Add docs torch.Tensor.requires_grad

docs/guides/model_convert/convert_from_pytorch/api_difference/Tensor/torch.Tensor.requires_grad.md

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+## [组合替代实现]torch.Tensor.requires_grad
+
+### [torch.Tensor.requires_grad](https://docs.pytorch.org/docs/stable/generated/torch.Tensor.requires_grad.html#torch-tensor-requires-grad)
+
+```python
+torch.Tensor.requires_grad
+```
+
+### [paddle.Tensor.stop_gradient](https://www.paddlepaddle.org.cn/documentation/docs/zh/develop/api/paddle/Tensor_cn.html#stop-gradient)
+
+```python
+paddle.Tensor.stop_gradient
+```
+
+Paddle 无此 API，需要组合实现。
+
+### 转写示例
+#### requires_grad：是否计算梯度
+```python
+# 当 torch 写法
+x.requires_grad = True
+
+# paddle 写法
+x.stop_gradient = False
+
+# 当 torch 写法
+x.requires_grad = False
+
+# paddle 写法
+x.stop_gradient = True
+```