Add slicing and int_repr() to QTensor (#19296)

Add slicing and int_repr() to QTensor (#19296)

Summary:
Stack:
    :black_circle:  **#19296 [pt1][quant] Add slicing and int_repr() to QTensor**  [:yellow_heart:](https://our.intern.facebook.com/intern/diff/D14756833/)
    :white_circle:  #18960 [pt1][quant] Add empty_quantized  [:yellow_heart:](https://our.intern.facebook.com/intern/diff/D14810261/)
    :white_circle:  #19312 Use the QTensor with QReLU  [:yellow_heart:](https://our.intern.facebook.com/intern/diff/D14819460/)
    :white_circle:  #19319 [RFC] Quantized SumRelu  [:yellow_heart:](https://our.intern.facebook.com/intern/diff/D14866442/)

Methods added to pytorch python frontend:
- int_repr() returns a CPUByte Tensor which copies the data of QTensor.
- Added as_strided for QTensorImpl which provides support for slicing a QTensor(see test_torch.py)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19296

Differential Revision: D14756833

Pulled By: jerryzh168

fbshipit-source-id: 6f4c92393330e725c4351d6ff5f5fe9ac7c768bf