replace np.frombuffer with torch.frombuffer in MNIST prototype (#4651)
replace np.frombuffer with torch.frombuffer in MNIST prototype (#4651)

* replace np.frombuffer with torch.frombuffer in MNIST prototype

* cleanup

* appease mypy

* more cleanup

* clarify inplace offset

* fix num bytes for floating point data
pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision