[Unity][Relax] Support Dynamic Tensor as Index, torch frontend

[guoyaol]

Add support: allow one dynamic tensor as torch.tensor's index

In Stable Diffusion XL:
e.g. output = input1 [input2.argmax(dim = -1), ]

Before: we can only support int, slice, elipsis, None
Now: we can have one dynamic torch.tensor in index, which allows us to represent SD XL correctly.

Behaviors aligns with torch.tensor:
as shown in test cases

Future TODO: Support other "fancy indexing" methods in torch.tensor, (e.g. multiple tensors as index) which requires some new relax ops

[Hzfengsy]

Thanks for the fix!

[guoyaol]

@junrushao @MasterJH5574