!9981 MindCon Bug Fix LearningRateScheduler Callback suzhou

From: @zhouyifengCode
Reviewed-by: 
Signed-off-by:

mindspore/train/callback/__init__.py

@@ -26,6 +26,7 @@ from ._checkpoint import ModelCheckpoint
 from ._loss_monitor import LossMonitor
 from ._time_monitor import TimeMonitor
 from ._summary_collector import SummaryCollector
+from ._lr_scheduler_callback import LearningRateScheduler
 
 __all__ = ["Callback", "LossMonitor", "TimeMonitor", "ModelCheckpoint",
-           "SummaryCollector", "CheckpointConfig", "RunContext"]
+           "SummaryCollector", "CheckpointConfig", "RunContext", "LearningRateScheduler"]

mindspore/train/callback/_lr_scheduler_callback.py

@@ -0,0 +1,69 @@
+# Copyright 2020 Huawei Technologies Co., Ltd
+#
+# 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.
+# ============================================================================
+"""LearningRateScheduler Callback class."""
+
+import math
+import numpy as np
+
+import mindspore.common.dtype as mstype
+from mindspore.common.tensor import Tensor
+from mindspore.train.callback._callback import Callback
+from mindspore.ops import functional as F
+
+class LearningRateScheduler(Callback):
+    """
+    Change the learning_rate during training.
+
+    Note:
+        This class are not supported on CPU.
+
+    Args:
+        learning_rate_function (Function): The function about how to change the learning rate during training.
+
+    Examples:
+        >>> from _lr_scheduler_callback import LearningRateScheduler
+        >>> import mindspore.nn as nn
+        >>> from mindspore.train import Model
+        ...
+        >>> def learning_rate_function(lr, cur_step_num):
+        ...     if cur_step_num%1000 == 0:
+        ...         lr = lr*0.1
+        ...     return lr
+        ...
+        >>> lr = 0.1
+        >>> momentum = 0.9
+        >>> net = Net()
+        >>> loss = nn.SoftmaxCrossEntropyWithLogits()
+        >>> optim = nn.Momentum(net.trainable_params(), learning_rate=lr, momentum=momentum)
+        >>> model = Model(net, loss_fn=loss, optimizer=optim)
+        ...
+        >>> dataset = create_custom_dataset("custom_dataset_path")
+        >>> model.train(1, dataset, callbacks=[LearningRateScheduler(learning_rate_function)],
+        ...             dataset_sink_mode=False)
+
+    """
+
+    def __init__(self, learning_rate_function):
+        super(LearningRateScheduler, self).__init__()
+        self.learning_rate_function = learning_rate_function
+
+    def step_end(self, run_context):
+        cb_params = run_context.original_args()
+        arr_lr = cb_params.optimizer.learning_rate.asnumpy()
+        lr = float(np.array2string(arr_lr))
+        new_lr = self.learning_rate_function(lr, cb_params.cur_step_num)
+        if not math.isclose(lr, new_lr, rel_tol=1e-10):
+            F.assign(cb_params.optimizer.learning_rate, Tensor(new_lr, mstype.float32))
+            print(f'At step {cb_params.cur_step_num}, learning_rate change to {new_lr}')