Add torch.scatter

[mavenlin]

• Aligned with the forward behavior of torch.scatter.
• Backward only works for index.shape == src.shape, due to torch's own problem.
• Add support to pass np.array directly for testing, because torch requests int64 for the index.
• Also add support for Torchish to take np.ndarray as per our discussion Torchish.__init__ take np.ndarray? #23.

[samuela]

thanks @mavenlin ! overall lgtm

[samuela]

thanks @mavenlin !