Update scnn for newer pytorch versions

[stephandooper]

later versions of pytorch have changed the torch.nn.grad backend. As a result, cpp files are no longer needed.

Question 1 : can this code be replaced with its original decorator @custom_fwd and @custom_bwd?

if '1.6' in torch.__version__: # type:ignore
    def forward_amp_decorator(func): 
        return torch.cuda.amp.custom_fwd(func)  # type:ignore
    def backward_amp_decorator(func): 
        return torch.cuda.amp.custom_bwd(func)  # type:ignore
    from torch.cuda.amp import autocast
else:
    def forward_amp_decorator(func): 
        return func
    def backward_amp_decorator(func):
        return func

Question 2: it seems that gather_gradient is now a dead end, ending at self.gradient being an empty dictionary and not being referenced anywhere. can it be removed?

StreamingCNN/scnn.py

Line 674 in 350df03

self.gradients = {}