[memory snapshot] track context for segments (#106113)
We want to display the stack for the original cudaMalloc that created a segment.
Previously we could only report the last time the segment memory was used,
or the record of the segment_alloc could appear in the list of allocator actions.
This PR ensure regardless of whether we still have the segment_alloc action,
the context for a segment is still available. The visualizer is updated to
be able to incorporate this information.
This PR adds a new field to Block. However the previous stacked cleanup PR
removed a field of the same size, making the change to Block size-neutral.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106113
Approved by: https://github.com/aaronenyeshi
