diff --git a/resnet.py b/resnet.py
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+++ b/resnet.py
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+# Copyright (c) 2019 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import division
+
+import argparse
+import contextlib
+import math
+import os
+import random
+
+import cv2
+import numpy as np
+
+import paddle
+import paddle.fluid as fluid
+from paddle.fluid.layer_helper import LayerHelper
+from paddle.fluid.dygraph.nn import Conv2D, Pool2D, BatchNorm, Linear
+
+from model import Model
+
+
+def center_crop_resize(img):
+    h, w = img.shape[:2]
+    c = int(224 / 256 * min((h, w)))
+    i = (h + 1 - c) // 2
+    j = (w + 1 - c) // 2
+    img = img[i: i + c, j: j + c, :]
+    return cv2.resize(img, (224, 224), 0, 0, cv2.INTER_LINEAR)
+
+
+def random_crop_resize(img):
+    height, width = img.shape[:2]
+    area = height * width
+
+    for attempt in range(10):
+        target_area = random.uniform(0.08, 1.) * area
+        log_ratio = (math.log(3 / 4), math.log(4 / 3))
+        aspect_ratio = math.exp(random.uniform(*log_ratio))
+
+        w = int(round(math.sqrt(target_area * aspect_ratio)))
+        h = int(round(math.sqrt(target_area / aspect_ratio)))
+
+        if w <= width and h <= height:
+            i = random.randint(0, height - h)
+            j = random.randint(0, width - w)
+            img = img[i: i + h, j: j + w, :]
+            return cv2.resize(img, (224, 224), 0, 0, cv2.INTER_LINEAR)
+
+    return center_crop_resize(img)
+
+
+def random_flip(img):
+    return img[:, ::-1, :]
+
+
+def normalize_permute(img):
+    # transpose and convert to RGB from BGR
+    img = img.astype(np.float32).transpose((2, 0, 1))[::-1, ...]
+    mean = np.array([123.675, 116.28, 103.53], dtype=np.float32)
+    std = np.array([58.395, 57.120, 57.375], dtype=np.float32)
+    invstd = 1. / std
+    for v, m, s in zip(img, mean, invstd):
+        v.__isub__(m).__imul__(s)
+    return img
+
+
+def compose(functions):
+    def process(sample):
+        img, label = sample
+        for fn in functions:
+            img = fn(img)
+        return img, label
+    return process
+
+
+def image_folder(path, shuffle=False):
+    valid_ext = ('.jpg', '.jpeg', '.png', '.ppm', '.bmp', '.webp')
+    classes = [d for d in os.listdir(path) if
+               os.path.isdir(os.path.join(path, d))]
+    classes.sort()
+    class_map = {cls: idx for idx, cls in enumerate(classes)}
+    samples = []
+    for dir in sorted(class_map.keys()):
+        d = os.path.join(path, dir)
+        for root, _, fnames in sorted(os.walk(d)):
+            for fname in sorted(fnames):
+                p = os.path.join(root, fname)
+                if os.path.splitext(p)[1].lower() in valid_ext:
+                    samples.append((p, class_map[dir]))
+    if shuffle:
+        random.shuffle(samples)
+
+    def iterator():
+        for s in samples:
+            yield s
+
+    return iterator
+
+
+class ConvBNLayer(fluid.dygraph.Layer):
+    def __init__(self,
+                 num_channels,
+                 num_filters,
+                 filter_size,
+                 stride=1,
+                 groups=1,
+                 act=None):
+        super(ConvBNLayer, self).__init__()
+
+        self._conv = Conv2D(
+            num_channels=num_channels,
+            num_filters=num_filters,
+            filter_size=filter_size,
+            stride=stride,
+            padding=(filter_size - 1) // 2,
+            groups=groups,
+            act=None,
+            bias_attr=False)
+
+        self._batch_norm = BatchNorm(num_filters, act=act)
+
+    def forward(self, inputs):
+        x = self._conv(inputs)
+        x = self._batch_norm(x)
+
+        return x
+
+
+class BottleneckBlock(fluid.dygraph.Layer):
+    def __init__(self,
+                 num_channels,
+                 num_filters,
+                 stride,
+                 shortcut=True):
+        super(BottleneckBlock, self).__init__()
+
+        self.conv0 = ConvBNLayer(
+            num_channels=num_channels,
+            num_filters=num_filters,
+            filter_size=1,
+            act='relu')
+        self.conv1 = ConvBNLayer(
+            num_channels=num_filters,
+            num_filters=num_filters,
+            filter_size=3,
+            stride=stride,
+            act='relu')
+        self.conv2 = ConvBNLayer(
+            num_channels=num_filters,
+            num_filters=num_filters * 4,
+            filter_size=1,
+            act=None)
+
+        if not shortcut:
+            self.short = ConvBNLayer(
+                num_channels=num_channels,
+                num_filters=num_filters * 4,
+                filter_size=1,
+                stride=stride)
+
+        self.shortcut = shortcut
+
+        self._num_channels_out = num_filters * 4
+
+    def forward(self, inputs):
+        x = self.conv0(inputs)
+        conv1 = self.conv1(x)
+        conv2 = self.conv2(conv1)
+
+        if self.shortcut:
+            short = inputs
+        else:
+            short = self.short(inputs)
+
+        x = fluid.layers.elementwise_add(x=short, y=conv2)
+
+        layer_helper = LayerHelper(self.full_name(), act='relu')
+        return layer_helper.append_activation(x)
+
+
+class ResNet(Model):
+    def __init__(self, depth=50, num_classes=1000):
+        super(ResNet, self).__init__()
+
+        layer_config = {
+            50: [3, 4, 6, 3],
+            101: [3, 4, 23, 3],
+            152: [3, 8, 36, 3],
+        }
+        assert depth in layer_config.keys(), \
+            "supported depth are {} but input layer is {}".format(
+                layer_config.keys(), depth)
+
+        layers = layer_config[depth]
+        num_channels = [64, 256, 512, 1024]
+        num_filters = [64, 128, 256, 512]
+
+        self.conv = ConvBNLayer(
+            num_channels=3,
+            num_filters=64,
+            filter_size=7,
+            stride=2,
+            act='relu')
+        self.pool = Pool2D(
+            pool_size=3,
+            pool_stride=2,
+            pool_padding=1,
+            pool_type='max')
+
+        self.blocks = []
+        for b in range(len(layers)):
+            shortcut = False
+            for i in range(layers[b]):
+                block = self.add_sublayer(
+                    'layer_{}_{}'.format(b, i),
+                    BottleneckBlock(
+                        num_channels=num_channels[b]
+                        if i == 0 else num_filters[b] * 4,
+                        num_filters=num_filters[b],
+                        stride=2 if i == 0 and b != 0 else 1,
+                        shortcut=shortcut))
+                self.blocks.append(block)
+                shortcut = True
+
+        self.global_pool = Pool2D(
+            pool_size=7, pool_type='avg', global_pooling=True)
+
+        stdv = 1.0 / math.sqrt(2048 * 1.0)
+        self.fc_input_dim = num_filters[len(num_filters) - 1] * 4 * 1 * 1
+        self.fc = Linear(self.fc_input_dim,
+                         num_classes,
+                         act='softmax',
+                         param_attr=fluid.param_attr.ParamAttr(
+                             initializer=fluid.initializer.Uniform(
+                                 -stdv, stdv)))
+
+    def forward(self, inputs):
+        x = self.conv(inputs)
+        x = self.pool(x)
+        for block in self.blocks:
+            x = block(x)
+        x = self.global_pool(x)
+        x = fluid.layers.reshape(x, shape=[-1, self.fc_input_dim])
+        x = self.fc(x)
+        return x
+
+
+def make_optimizer(parameter_list=None):
+    total_images = 1281167
+    base_lr = 0.1
+    momentum = 0.9
+    l2_decay = 1e-4
+
+    step_per_epoch = int(math.floor(float(total_images) / FLAGS.batch_size))
+    boundaries = [step_per_epoch * e for e in [30, 60, 80]]
+
+    lr = [base_lr * (0.1**i) for i in range(len(boundaries) + 1)]
+    optimizer = fluid.optimizer.Momentum(
+        learning_rate=fluid.layers.piecewise_decay(
+            boundaries=boundaries, values=lr),
+        momentum=momentum,
+        regularization=fluid.regularizer.L2Decay(l2_decay),
+        parameter_list=parameter_list)
+    return optimizer
+
+
+def accuracy(pred, label, topk=(1, )):
+    maxk = max(topk)
+    pred = np.argsort(pred)[:, ::-1][:, :maxk]
+    correct = (pred == np.repeat(label, maxk, 1))
+
+    batch_size = label.shape[0]
+    res = []
+    for k in topk:
+        correct_k = correct[:, :k].sum()
+        res.append(100.0 * correct_k / batch_size)
+    return res
+
+
+def run(model, loader, mode='train'):
+    total_loss = 0.0
+    total_acc1 = 0.0
+    total_acc5 = 0.0
+    num_steps = 0
+    device_ids = list(range(FLAGS.num_devices))
+    for idx, batch in enumerate(loader()):
+        outputs, losses = getattr(model, mode)(
+            batch[0], batch[1], device='gpu', device_ids=device_ids)
+        top1, top5 = accuracy(outputs[0], batch[1], topk=(1, 5))
+
+        total_loss += np.sum(losses)
+        total_acc1 += top1
+        total_acc5 += top5
+        num_steps += 1
+        if idx % 10 == 0:
+            print("{:04d}: loss {:0.3f} top1: {:0.3f}% top5: {:0.3f}%".format(
+                idx, total_loss / num_steps,
+                total_acc1 / num_steps, total_acc5 / num_steps))
+        num_steps += 1
+
+
+def main():
+    @contextlib.contextmanager
+    def null_guard():
+        yield
+
+    epoch = FLAGS.epoch
+    batch_size = FLAGS.batch_size
+    if FLAGS.dynamic:
+        guard = fluid.dygraph.guard()
+    else:
+        guard = null_guard()
+
+    train_dir = os.path.join(FLAGS.data, 'train')
+    val_dir = os.path.join(FLAGS.data, 'val')
+
+    train_loader = fluid.io.xmap_readers(
+        lambda batch: (np.array([b[0] for b in batch]),
+                       np.array([b[1] for b in batch]).reshape(-1, 1)),
+        paddle.batch(
+            fluid.io.xmap_readers(
+                compose([cv2.imread, random_crop_resize, random_flip,
+                         normalize_permute]),
+                image_folder(train_dir, shuffle=True),
+                process_num=8,
+                buffer_size=4 * batch_size),
+            batch_size=batch_size,
+            drop_last=True),
+        process_num=2, buffer_size=4)
+
+    val_loader = fluid.io.xmap_readers(
+        lambda batch: (np.array([b[0] for b in batch]),
+                       np.array([b[1] for b in batch]).reshape(-1, 1)),
+        paddle.batch(
+            fluid.io.xmap_readers(
+                compose([cv2.imread, center_crop_resize, normalize_permute]),
+                image_folder(val_dir),
+                process_num=8,
+                buffer_size=4 * batch_size),
+            batch_size=batch_size),
+        process_num=2, buffer_size=4)
+
+    if not os.path.exists('checkpoints'):
+        os.mkdir('checkpoints')
+
+    with guard:
+        model = ResNet()
+        sgd = make_optimizer(parameter_list=model.parameters())
+        model.prepare(sgd, 'cross_entropy')
+
+        for e in range(epoch):
+            print("======== train epoch {} ========".format(e))
+            run(model, train_loader)
+            model.save('checkpoints/{:02d}'.format(e))
+            print("======== eval epoch {} ========".format(e))
+            run(model, val_loader, mode='eval')
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser("Resnet Training on ImageNet")
+    parser.add_argument('data', metavar='DIR', help='path to dataset '
+                        '(should have subdirectories named "train" and "val"')
+    parser.add_argument(
+        "-e", "--epoch", default=90, type=int, help="number of epoch")
+    parser.add_argument(
+        "-b", "--batch_size", default=512, type=int, help="batch size")
+    parser.add_argument(
+        "-n", "--num_devices", default=4, type=int, help="number of devices")
+    parser.add_argument(
+        "-d", "--dynamic", action='store_true', help="enable dygraph mode")
+    FLAGS = parser.parse_args()
+    main()