expandable_segments fix possible assert (#106818)
If record_history is enabled, then a block is allocated, record_history
is disabled, and then the block is freed and later unnmapped, we can hit
the `to_map->context_when_allocated == nullptr` assertion.
This change universally clears context_when_allocated on free, which should
prevent this sequence of events from happening.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106818
Approved by: https://github.com/eellison
