[c10d] Start deprecating *_coalesced APIs (#85959)
- We consider that general users need not to use the `*_coalesced` APIs unless there is an extreme concern about performance.
- We are investigating using a context manager named `coalescing_manager` which wrap around multiple individual collectives to compose the coalescing hint, rather than giving each collective a *_coalesced variant.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85959
Approved by: https://github.com/XilunWu, https://github.com/H-Huang
