**Summary**
1. This PR removes the public API `compute_local_shape` and replace its use with the more general API `compute_local_shape_and_global_offset`.
2. To keep `compute_local_shape_and_global_offset` consistent with `compute_local_shape` on empty shards, it now returns local tensor shape `(0,)` for empty shards which is more aligned with DTensor's semantics on non-participating ranks.
**Test**
`pytest test/distributed/_tensor/test_dtensor.py`
`pytest test/distributed/_tensor/test_init.py`
`pytest test/distributed/_tensor/test_tensor_ops.py`
Differential Revision: [D62415591](https://our.internmc.facebook.com/intern/diff/D62415591)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/135554
Approved by: https://github.com/tianyu-l, https://github.com/wz337
reland of https://github.com/pytorch/pytorch/pull/133113
I have to create a new PR because the previous reverted PR could not either be rebased, or imported successfully :(
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Moving DTensor to be in the public namespace, to formally add the documentation page that includes all the public APIs. This includes:
* many path renames and path import fixes
* a dedicated doc page without too much content yet (adding in the next PRs)
* To preserve the BC for users still using the torch.distributed._tensor, I added a shim script to redirect old path calls to the new module
The BC preserving is evidented by the fact that all DTensor tests are still working without changing the public imports. So it's safe to land the changes
Pull Request resolved: https://github.com/pytorch/pytorch/pull/134203
Approved by: https://github.com/tianyu-l
