Reland of #172610: same code as previous land except:

• includes fix redistribute() handling for finding flattened device mesh dims under compile #173873 (credit @bdhirsh)
• includes [dtensor] fix flatten mesh dims arg relative to submesh #173790 (credit @IvanKobzarev)
• includes [DTensor] make debugmode print optimized transforminfos #173436
• adds disable contextmanager + test

Ensures that when possible (when such a flattened mesh exists), DTensor will find and use it to avoid more costly sequential comms, and particularly for reduce comms, also avoids the risk of different reduction orders causing divergent results. (See this doc for more info.

Example: For a (2,2,2) mesh with dims (A,B,C) and placements
when redistributing from (Psum, Replicate, Psum) -> (Replicate,
Replicate, Replicate) - the original behavior would be 2 separate
all_reduces. After this PR, if the user flattens dims A,C, this becomes
one larger all_reduce.

Compared with earlier attempt #172119, this PR

• includes optimization for comms other than all_reduce
• explicitly bans mixed partial types (Psum, Pmax) is not a valid placement, so we don't have to worry about optimizing around it
• therefore uses a simpler implementation involving grouping adjacent transforminfos and then merging like kinds
• Warns once per mesh shape for missing flattened meshes
• Won't optimize reduce_scatters when they shard an uneven sized tensor dim

Details/Limitations

• all_to_all is never merged (left for possible future work, but not obvious how to do it in general)
• reduce_scatter is only merged when the outermost partial shape is evenly divisible by the flattened mesh - otherwise, warns
• reduce_scatter and all_gather are only merged when the shards are in left-to-right (ascending) order, since DeviceMesh only supports flattening in ascending order and the mesh ordering impacts correctness.
• groups of like-kind collectives are NOT combined if they are not adjacent in the transform_info list
• flattened device-meshes are not automatically created due to preference of explicit creation and ensuring torch.compile works, but warnings prompt the user to create them when it would help allow an optimization
• DOES support merging mixed Partial (sum, avg) reductions, using the product of the avg dim sizes to scale after performing a sum reduction on the merged mesh. Refuses to merge any other combinations of mixed partials.

Fixes #171916

Note: initial attempt used stable sort with a lt
method in TransformInfo comparing comm type key, but this was not correct because sorting a
local (no-comm) operation like chunking before or after a comm operation
on the same mesh time affects results.

Differential Revision: D92540256