[C/PyTorch] Refactor and move userbuffers into TE/common

[denera]

This PR moves all the userbuffers code in TE/pytorch to TE/common and refactors the interfaces to make TE/common/userbuffers accessible to all framework integrations.

To do:

• [x] Move userbuffers from TE/pytorch to TE/common.
• [x] Bootstrap userbuffers with PyTorch collectives.
• [x] Update build logic with CXX ABI version fix and correct rpaths.
• [x] Implement comm overlap example for PyTorch.
• [x] Verify split_overlap_ag_p2p
• [x] Verify split_overlap_rs_p2p
• [ ] Verify split_overlap_rs
• [ ] Verify atomic_gemm_overlap_ag_p2p
• [ ] Verify atomic_gemm_overlap_rs_p2p
• [ ] Verify atomic_gemm_overlap_rs
• [ ] Verify bulk_overlap for AG
• [ ] Verify bulk_overlap for RS
• [ ] Implement unit tests.

[denera]

@timmoon10 FYI I will be removing the 3rd party dlpack package I introduced earlier in this PR. It's not needed for the PyTorch collective callbacks, and I can bring it back if it becomes necessary for JAX down the line (but I'd like to avoid it if I can).

[denera]

This work has been moved to a new branch due to too many conflicts with TE/main. Closing the PR and filing a new one.