[memory snapshots] removed chained history (#106079)
For free blocks of memory in the allocator, we previously kept a linked list
of the stack frames of previous allocations that lived there. This was only
ever used in one flamegraph visualization and never proved useful at
understanding what was going on. When memory history tracing was added, it
became redundant, since we can see the history of the free space from recording
the previous actions anyway.
This patch removes this functionality and simplifies the snapshot format:
allocated blocks directly have a 'frames' attribute rather than burying stack frames in the history.
Previously the memory history tracked the real size of allocations before rounding.
Since history was added, 'requested_size' has been added directly to the block which records the same information,
so this patch also removes that redundancy.
None of this functionality has been part of a PyTorch release with BC guarentees, so it should be safe to alter
this part of the format.
This patch also updates our visualization tools to work with the simplified format. Visualization tools keep
support for the old format in `_legacy` functions so that during the transition old snapshot files can still be read.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106079
Approved by: https://github.com/eellison
