add initilaizer (#5494)

* add initilaizer

* fix doc

tutorials/article-implementation/initializer.md

+
+# 模型参数初始化对齐方法
+
+# 1. 背景
+
+Paddle提供了大量的初始化方法，包括`Constant`, `KaimingUniform`, `KaimingNormal`,  `TruncatedNormal`,  `Uniform`,  `XavierNormal`, `XavierUniform`等，合适的初始化方法能够帮助模型快速地收敛或者达到更高的精度。
+
+论文复现的过程中，在训练对齐环节，需要保证Paddle的复现代码和参考代码保持一致，从而实现完全对齐。然而由于不同框架的差异性，部分API中参数提供的默认初始化方法有区别，该文档以`nn.Conv2D`以及`nn.Linear`这两个最常用的API为例，介绍怎样实现对齐。
+
+**更多参考链接：**
+
+* Paddle初始化相关API链接：[初始化API官网文档](https://www.paddlepaddle.org.cn/documentation/docs/zh/api/paddle/nn/Overview_cn.html#chushihuaxiangguan)
+* Paddle提供的初始化方式为直接修改API的`ParamAttr`，与`torch.nn.init`等系列API的使用方式不同，PaddleDetection中实现了与`torch.nn.init`系列API完全对齐的初始化API，包括`uniform_`, `normal_`, `constant_`, `ones_`, `zeros_`, `xavier_uniform_`, `xavier_normal_`, `kaiming_uniform_`, `kaiming_normal_`, `linear_init_`, `conv_init_`，可以参考[initializer.py](https://github.com/PaddlePaddle/PaddleDetection/blob/develop/ppdet/modeling/initializer.py)，查看更多的实现细节。
+
+
+# 2. 不同框架的初始化差异
+
+## 2.1 默认初始化的对齐方法
+
+在此情况下，一般需要查看文档，了解参考代码的初始化方法，从而通过修改初始化方法，实现初始化的对齐。
+
+下面以`nn.Conv2D` API为例进行说明。
+
+* **Step1：** 基于Paddle与torch，定义2个卷积操作，绘制其weight参数的直方图，如下所示。
+
+```python
+import paddle
+import torch
+import numpy as np
+import matplotlib.pyplot as plt
+%matplotlib inline
+
+conv2d_pd = paddle.nn.Conv2D(4096, 512, 3)
+conv2d_pt = torch.nn.Conv2d(4096, 512, 3)
+
+conv2d_pd_weight = conv2d_pd.weight.numpy().reshape((-1, ))
+conv2d_pt_weight = conv2d_pt.weight.detach().numpy().reshape((-1, ))
+plt.figure(figsize=(10, 6))
+temp = plt.hist([conv2d_pd_weight, conv2d_pt_weight], bins=100, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Conv2D weight", "torch.nn.Conv2d weight"})
+```
+
+<div align="center">
+<img src="https://paddle-model-ecology.bj.bcebos.com/images/initializer/conv2d_weight_default_diff.jpeg"  width = "600" />
+</div>
+
+结合[paddle文档](https://www.paddlepaddle.org.cn/documentation/docs/zh/api/paddle/ParamAttr_cn.html#paramattr)和[torch文档](https://pytorch.org/docs/stable/generated/torch.nn.Conv2d.html?highlight=conv2d#torch.nn.Conv2d)可知，paddle的初始化是`XavierNormal`，torch的初始化是`uniform`，初始化方法边界值是`(-sqrt(groups/(in_channels*prod(*kernal_size))), sqrt(groups/(in_channels*prod(*kernal_size))))`。
+
+
+* **Step2：** 由上述分析，基于`paddle.nn.initializer.Uniform` API，自定义Paddle中Conv2D的初始化，代码如下所示：
+
+```python
+import paddle
+import torch
+import numpy as np
+import matplotlib.pyplot as plt
+import paddle.nn.initializer as init
+%matplotlib inline
+# 该例子中，对应上述公式的group=1，in_channels=4096，kernal_size=3,由于二维卷积的卷积核是二维的，所以此处的结果为4096*3*3
+conv2d_pd = paddle.nn.Conv2D(4096, 512, 3,
+            init.Uniform(-1/math.sqrt(4096*3*3), 1/math.sqrt(4096*3*3)))
+conv2d_pt = torch.nn.Conv2d(4096, 512, 3)
+
+conv2d_pd_weight = conv2d_pd.weight.numpy().reshape((-1, ))
+conv2d_pt_weight = conv2d_pt.weight.detach().numpy().reshape((-1, ))
+plt.figure(figsize=(10, 6))
+temp = plt.hist([conv2d_pd_weight, conv2d_pt_weight], bins=100, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Conv2D weight", "torch.nn.Conv2d weight"})
+```
+
+<div align="center">
+<img src="https://paddle-model-ecology.bj.bcebos.com/images/initializer/conv2d_weight_fixed_diff.jpeg"  width = "600" />
+</div>
+
+从图中可知，二者的初始化参数分布实现一致。
+
+
+## 2.2 自定义初始化的对齐方法
+
+
+部分参考代码中，初始化的方法是通过使用`torch.nn.init`系列API实现，可以认为是自定义初始化。例如：[resnet](https://github.com/pytorch/vision/blob/ec1c2a12cf00c6df83c7fb88f75b8117cda2f970/torchvision/models/resnet.py#L208)中使用的`kaiming_normal_`传入了`mode`和`nonlinearity`两个参数：
+
+```python
+if isinstance(m, nn.Conv2d):
+    nn.init.kaiming_normal_(m.weight, mode="fan_out", nonlinearity="relu")
+```
+
+在这类问题中，可以先尝试使用`2.1`章节中的`Step1`，查看使用Paddle同名初始化方式的默认参数是否能够对齐。如果无法对齐，可以查阅[initializer.py](https://github.com/PaddlePaddle/PaddleDetection/blob/develop/ppdet/modeling/initializer.py)，使用该文件中的初始化函数，实现对齐。
+
+不同框架的初始化方法有所不同，开发者论文复现过程中难以排查，因此下面第3章介绍通过自定义初始化的方式，实现不同框架的参数初始化分布一致，最终帮助大家更加顺利地完成论文复现。
+
+**注意：** BatchNorm2D等大多数的API中，可学习参数的初始化分布相同，在此为进一步对比，也给出其权重的可视化对比图像。
+
+# 3. 初始化参数分布对比
+
+## 3.1 默认初始化不同的API权重直方图对比
+
+| Paddle API | torch API  | 默认初始化方法的参数分布对比图 | 修改初始化参数方法 | 修改之后的参数分布对比 |
+|:---------:|:------------------:|:------------:|:------------:|:------------:|
+| `paddle.nn.Conv2D` weight参数 | `torch.nn.Conv2d` weight参数 | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/conv2d_weight_default_diff.jpeg) | 见附录`4.1`  | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/conv2d_weight_fixed_diff.jpeg) |
+| `paddle.nn.Conv2D` bias参数 | `torch.nn.Conv2d` bias参数 | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/conv2d_bias_default_diff.jpeg) | 见附录`4.1`  | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/conv2d_bias_fixed_diff.jpeg) |
+| `paddle.nn.Linear` weight参数 | `torch.nn.Linear` weight参数 | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/linear_weight_default_diff.jpeg) | 见附录`4.2`  | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/linear_weight_fixed_diff.jpeg) |
+| `paddle.nn.Linear` bias参数 | `torch.nn.Linear` bias参数 | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/linear_bias_default_diff.jpeg) | 见附录`4.2`  | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/linear_bias_fixed_diff.jpeg) |
+
+
+
+## 3.2 默认初始化相同的API权重直方图对比
+
+| Paddle API | torch API  | 默认初始化方法的参数分布对比图 |
+|:---------:|:------------------:|:------------:|
+| `paddle.nn.BatchNorm2D` weight参数 | `torch.nn.BatchNorm2d` weight参数 | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/bn_weight_default_diff.jpeg) |
+| `paddle.nn.BatchNorm2D` bias参数 | `torch.nn.BatchNorm2d` bias参数 | ![](https://paddle-model-ecology.bj.bcebos.com/images/initializer/bn_bias_default_diff.jpeg) |
+
+# 4. 附录
+
+## 4.1 初始化对齐代码
+
+### 4.1.1 paddle.nn.Conv2D
+
+* 默认初始化以及可视化代码
+
+```python
+import paddle
+import torch
+import numpy as np
+import matplotlib.pyplot as plt
+%matplotlib inline
+
+conv2d_pd = paddle.nn.Conv2D(4096, 512, 3)
+conv2d_pt = torch.nn.Conv2d(4096, 512, 3)
+
+conv2d_pd_weight = conv2d_pd.weight.numpy().reshape((-1, ))
+conv2d_pd_bias = conv2d_pd.bias.numpy().reshape((-1, ))
+conv2d_pt_weight = conv2d_pt.weight.detach().numpy().reshape((-1, ))
+conv2d_pt_bias = conv2d_pd.bias.numpy().reshape((-1, ))
+plt.figure(figsize=(10, 6))
+temp = plt.hist([conv2d_pd_weight, conv2d_pt_weight], bins=100, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Conv2D weight", "torch.nn.Conv2d weight"})
+
+plt.figure(figsize=(10, 6))
+temp = plt.hist([conv2d_pd_bias, conv2d_pt_bias], bins=50, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Conv2D bias", "torch.nn.Conv2d bias"})
+```
+
+
+
+* 修正后初始化以及可视化代码
+
+```python
+import paddle
+import torch
+import numpy as np
+import matplotlib.pyplot as plt
+import paddle.nn.initializer as init
+%matplotlib inline
+
+conv2d_pd = paddle.nn.Conv2D(4096, 512, 3,
+            weight_attr=init.Uniform(-1/math.sqrt(4096*3*3), 1/math.sqrt(4096*3*3)),
+            bias_attr=init.Uniform(-1/math.sqrt(4096*3*3), 1/math.sqrt(4096*3*3)))
+conv2d_pt = torch.nn.Conv2d(4096, 512, 3)
+
+conv2d_pd_weight = conv2d_pd.weight.numpy().reshape((-1, ))
+conv2d_pd_bias = conv2d_pd.bias.numpy().reshape((-1, ))
+conv2d_pt_weight = conv2d_pt.weight.detach().numpy().reshape((-1, ))
+conv2d_pt_bias = conv2d_pd.bias.numpy().reshape((-1, ))
+plt.figure(figsize=(10, 6))
+temp = plt.hist([conv2d_pd_weight, conv2d_pt_weight], bins=100, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Conv2D weight", "torch.nn.Conv2d weight"})
+
+plt.figure(figsize=(10, 6))
+temp = plt.hist([conv2d_pd_bias, conv2d_pt_bias], bins=50, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Conv2D bias", "torch.nn.Conv2d bias"})
+```
+
+
+### 4.1.2 paddle.nn.Linear
+
+* 默认初始化以及可视化代码
+
+```python
+import paddle
+import torch
+import numpy as np
+import matplotlib.pyplot as plt
+%matplotlib inline
+
+linear_pd = paddle.nn.Linear(4096, 512)
+linear_pt = torch.nn.Linear(4096, 512)
+
+linear_pd_weight = linear_pd.weight.numpy().reshape((-1, ))
+linear_pd_bias = linear_pd.bias.numpy().reshape((-1, ))
+linear_pt_weight = linear_pt.weight.detach().numpy().reshape((-1, ))
+linear_pt_bias = linear_pt.bias.numpy().reshape((-1, ))
+plt.figure(figsize=(10, 6))
+temp = plt.hist([linear_pd_weight, linear_pt_weight], bins=100, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Linear weight", "torch.nn.Linear weight"})
+
+plt.figure(figsize=(10, 6))
+temp = plt.hist([linear_pd_bias, linear_pt_bias], bins=50, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Linear bias", "torch.nn.Linear bias"})
+```
+
+* 修正后初始化以及可视化代码
+
+```python
+import paddle
+import torch
+import numpy as np
+import matplotlib.pyplot as plt
+%matplotlib inline
+# linear的初始化方法同样适用于2.1节中的公式，此处的kernal_size等价于1。
+linear_pd = paddle.nn.Linear(4096, 512,
+            weight_attr=init.Uniform(-1/math.sqrt(4096), 1/math.sqrt(4096)),
+            bias_attr=init.Uniform(-1/math.sqrt(4096), 1/math.sqrt(4096)))
+linear_pt = torch.nn.Linear(4096, 512)
+
+linear_pd_weight = linear_pd.weight.numpy().reshape((-1, ))
+linear_pd_bias = linear_pd.bias.numpy().reshape((-1, ))
+linear_pt_weight = linear_pt.weight.detach().numpy().reshape((-1, ))
+linear_pt_bias = linear_pt.bias.numpy().reshape((-1, ))
+plt.figure(figsize=(10, 6))
+temp = plt.hist([linear_pd_weight, linear_pt_weight], bins=100, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Linear weight", "torch.nn.Linear weight"})
+
+plt.figure(figsize=(10, 6))
+temp = plt.hist([linear_pd_bias, linear_pt_bias], bins=50, rwidth=0.8, histtype="step")
+plt.xlabel("value")
+plt.ylabel("count")
+plt.legend({"paddle.nn.Linear bias", "torch.nn.Linear bias"})
+```