[DTensor] Fix _get_or_create_default_group() (#96961)
Summary:
This PR fixes `_get_or_create_default_group()` of `DeviceMesh`. When `mesh` of the first created `DeviceMesh` is not `[0, 1, 2, ... WORLD_SIZE - 1]` and `is_initialized() == False`, it wrongly asserts. This PR fixes this issue by removing these assertions.
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More specifically, `_get_or_create_default_group()` has 4 checks:
1. `DeviceMesh must include every process in WORLD`
2. `DeviceMesh cannot have duplicate values`
3. `DeviceMesh ranks must start from 0`
4. `DeviceMesh should have all ranks of WORLD`
1, 3, and 4 are not satisfied when `self.mesh` is not `[0, 1, 2, ... WORLD_SIZE - 1]`.
2 is a valid check, but it is also checked in `__init__()`, so we don't need to check it again in this function.
Test Plan: CI
Reviewed By: wanchaol
Differential Revision: D44098849
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96961
Approved by: https://github.com/wanchaol
