add optim (#927)

docs/pytorch_project_convertor/API_docs/optimizer/README.md

+## 优化器与学习率API映射列表
+
+该文档梳理了与优化器与学习率相关的PyTorch-PaddlePaddle API映射列表。  
+
+| 序号 | PyTorch API                                                  | PaddlePaddle API                                             | 备注                                                    |
+| ---- | ------------------------------------------------------------ | ------------------------------------------------------------ | ------------------------------------------------------- |
+| 1    | [torch.optim.lr_scheduler.LRScheduler](https://pytorch.org/docs/master/_modules/torch/optim/lr_scheduler.html#LRScheduler) | [paddle.optimizer.lr.LRScheduler](https://www.paddlepaddle.org.cn/documentation/docs/zh/api/paddle/optimizer/lr/LRScheduler_cn.html) | [差异对比](https://github.com/PaddlePaddle/X2Paddle/tree/develop/docs/pytorch_project_convertor/API_docs/optimizer/torch.optim.lr_scheduler.LRScheduler.md)

docs/pytorch_project_convertor/API_docs/optimizer/torch.optim.lr_scheduler.LRScheduler.md

+## torch.optim.lr_scheduler.LRScheduler
+
+### [torch.optim.lr_scheduler.LRScheduler](https://pytorch.org/docs/master/_modules/torch/optim/lr_scheduler.html#LRScheduler)
+
+```python
+torch.optim.lr_scheduler.LRScheduler(optimizer, last_epoch=-1, verbose=False)
+```
+
+### [paddle.optimizer.lr.LRScheduler](https://www.paddlepaddle.org.cn/documentation/docs/zh/api/paddle/optimizer/lr/LRScheduler_cn.html)
+
+```python
+paddle.optimizer.lr.LRScheduler(learning_rate=0.1, last_epoch=- 1, verbose=False)
+```
+
+### 参数差异
+
+| PyTorch       | PaddlePaddle  | 备注                                                   |
+| ------------- | ------------- | ----------------------------------------------------- |
+| optimizer     | -             | 优化器, PaddlePaddle 无此参数。                          |
+| -             | learning_rate | 学习率, Pytorch 无此参数。                               |
+
+### 使用差异
+
+#### PyTorch
+
+```python
+import torch
+import torch.nn as nn
+import torch.optim as optim
+from torch.optim import lr_scheduler
+
+model = nn.Linear(10, 10)
+optimizer = optim.SGD(model.parameters(), lr=0.1)
+scheduler = lr_scheduler.StepLR(optimizer, step_size=30, gamma=0.1)
+for epoch in range(100):
+    train(...)
+    validate(...)
+    scheduler.step()
+```
+
+#### PaddlePaddle
+
+```python
+import paddle
+import paddle.nn as nn
+import paddle.optimizer as optim
+from paddle.optimizer.lr import StepDecay
+
+model = nn.Linear(10, 10)
+scheduler = StepDecay(learning_rate=0.1, step_size=30, gamma=0.1)
+optimizer = optim.SGD(learning_rate=scheduler, parameters=model.parameters())
+
+for epoch in range(100):
+    train(...)
+    validate(...)
+    scheduler.step()
+```