message: Update README to fit PyTorch 1.7
diff: @@ -14,13 +14,13 @@ Intel Extension for PyTorch is a Python package to extend official PyTorch. It i
  ### Install PyTorch from Source
  - 1. Get PyTorch v1.5.0-rc3 source(Refer to [PyTorch guide](https://github.com/pytorch/pytorch#get-the-pytorch-source) for more details)
  + 1. Get PyTorch v1.7.0 source(Refer to [PyTorch guide](https://github.com/pytorch/pytorch#get-the-pytorch-source) for more details)
  ```bash
  git clone --recursive https://github.com/pytorch/pytorch
  cd pytorch
  # checkout source code to the specified version
  - git checkout v1.5.0-rc3
  + git checkout v1.7.0
  # update submodules for the specified PyTorch version
  git submodule sync
  @@ -41,7 +41,7 @@ Intel Extension for PyTorch is a Python package to extend official PyTorch. It i
  ```bash
  # Apply git patch to pytorch code
  cd ${pytorch_directory}
  - git apply ${intel_extension_for_pytorch_directory}/torch_patches/dpcpp-v1.5-rc3.patch
  + git apply ${intel_extension_for_pytorch_directory}/torch_patches/xpu-1.7.patch
  ```
  4. Build and install PyTorch (Refer to [PyTorch guide](https://github.com/pytorch/pytorch#install-pytorch) for more details)
  