Revert D113962248: Optimizer scratch-buffer stashing in MemoryStashingManager (#4495)

[doIIarplus]

Summary:

D113962248 added the torchrec half of "free DistributedShampoo scratch buffers
during the optimizer-state stash window": the _ScratchBufferOptimizer protocol,
scratch_buffers() discovery, _release_optimizer_scratch_buffers, a second
_optimizer_scratch_buffer_restore_callbacks list, the
restore_optimizer_state(restore_scratch_buffer=...) parameter, and
scratch_buffers() fan-out through CombinedOptimizer / KeyedOptimizerWrapper
/ OptimizerWrapper / SemiSyncOptimizer.

None of that machinery is needed. _global_dist_buffer is pure scratch that
DistributedShampoo allocates, fills, and consumes entirely within
update_params(); it never leaves the optimizer and is not part of state_dict.
Routing its free/re-alloc through torchrec meant a cross-repo protocol plus
wrapper fan-out and a parallel callback list, purely so an external caller could
drive a resize the owner could do itself.

Reverting so the feature can be re-implemented inside
hpc/optimizers/distributed_shampoo/dev/ behind
DDPDistributedConfig.free_dist_buffer_between_steps.

Middle of a 3-diff revert stack (on top of the D113962249 revert).

Two deliberate carve-outs -- this is NOT a mechanical sl backout:

1. Kept the BUCK dep fix. D113962248 also changed //caffe2:_torch ->
   fbsource//third-party/pypi/torch:torch in torchrec/optim/BUCK and
   torchrec/optim/tests/BUCK. _torch is a private target that should not be
   depended on directly, so those hunks are left in place.

2. Kept the staged_cpu_view_for contiguity fix and its TestCheckpointWhileStashed
   tests. D113962248 also replaced the view.set_(storage, offset, shape, stride)
   reconstruction with a contiguous reshape/narrow/view. That is an unrelated
   correctness fix: chunked_copy_ fills the pinned buffer in logical/row-major
   order, so rebuilding the view with the source's original (possibly transposed)
   stride hands the DCP stager transposed values -> silently corrupt optimizer
   state on resume. Reverting it would reintroduce that bug.

Original commit changeset: 37123fd3d19f
Original Phabricator Diff: D113962248

Reviewed By: wz337, hjmshi

Differential Revision: D114959521

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[wz337]

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