Fix deleting destination buffers during reshard.

[NicoGrande]

This PR cleans up some of the buffer deletion code in the transfer_state_directly codepath and adds additional guardrails to avoid deleting aliased tensors.

Additionally this PR introduces new unittests for previously untested codepaths like deleting buffers when reshard chunking is explicitly enabled.

Checklist

• I have added all the necessary unit tests for my change.
• I have verified that my change does not break existing code and all unit tests pass.
• I have added all appropriate doc-strings/documentation.
• My PR is based on the latest changes of the main branch (if unsure, rebase the code).
• I have signed the Contributor License Agreement.
• I have followed Contribution Guidelines.