Add error message for shape mismatch

[xavierforge]

Address for #14 , catch the shape mismatch before PyTorch throws RuntimeError. Returns an error message as below:

AssertionError: Two tensors have a different shapes
 Spec: 
	Expect: torch.Size([5]) 
	Got: torch.Size([1, 5])

[srush]

Sorry, this unfortunately breaks broadcasting. You could try catching the runtime error.

[xavierforge]

Sorry, I didn't notice this would break broadcasting. I've changed the assertion only for this particular runtime error.