Avoid using einsum for torch.cat DTensor propogation (#100251)
DTensor was reusing `einop_rule` to propagate sharding for torch.cat.
However, einsum only supports up to 52 subscripts (i.e., input tensors).
We have encountered use cases where one cat operator has more than 60
input tensors. Therefore, this commit reimplements sharding prop
rule for cat without using einsum.
Differential Revision: [D45435232](https://our.internmc.facebook.com/intern/diff/D45435232)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100251
Approved by: https://github.com/wanchaol
