Add tracer implementation

test=develop

python/paddle/fluid/imperative/tracer.py

+# Copyright (c) 2018 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 print_function
+
+import six
+
+from collections import defaultdict
+from paddle.fluid import core
+from paddle.fluid import framework
+
+__all__ = ['Tracer']
+
+
+def release_op(op):
+    del framework._imperative_tracer()._ops[op._trace_id]
+
+
+class Tracer(core.Tracer):
+    """
+    Python wrapper of imperative tracer
+    """
+
+    def __init__(self, block):
+        super(Tracer, self).__init__(block)
+
+        self._ops = defaultdict()
+        self._trace_id = 0
+
+    def trace_op(self, op, stop_gradient=False):
+        # record op's trace id
+        op.iop._trace_id = self._trace_id
+        self._trace_id += 1
+
+        # trace op and save it
+        backward_refs = self.trace(op.iop, op.inputs, op.outputs, op.block.desc,
+                                   framework._current_expected_place(),
+                                   stop_gradient)
+
+        if not stop_gradient:
+            self._ops[op.iop._trace_id] = op
+
+            # register backward hooks and variables if needed
+            if len(backward_refs) > 0:
+                op.iop.register_backward_hooks(release_op)
+
+                op.backward_refs = defaultdict(list)
+                for k, v in six.iteritems(op.inputs):
+                    if k in backward_refs:
+                        op.backward_refs[k] = op.inputs[k]
+
+                for k, v in six.iteritems(op.outputs):
+                    if k in backward_refs:
+                        op.backward_refs[k] = op.outputs[k]