Add a new experimental option jax_pmap_no_rank_reduction.
This option changes the implementation of pmap so that the individual shards have the same rank as the entire array, i.e. in the terminology of pmap using a "chunked" axis instead of an "unstacked" axis.
i.e., previously a typical array used by pmap might have a shape of, say, [8, 100], if sharded across 8 accelerators on its first axis, and each individual shard would have a shape of, say, [100]. With this change, each individual shard has a shape of [1, 100] instead.
Why do this?
The main reason to do this is that XLA's sharding (HloSharding), which is exposed in JAX as GSPMDSharding/NamedSharding/PositionalSharding, cannot represent a change of rank. This means that the kind of sharding used by pmap cannot be represented to XLA as a sharding. If we change the definition of PmapSharding to preserve the array rank instead, then this means that PmapSharding can in the future be represented directly as a kind of sharding known to XLA.
The new definition of PmapSharding will allow a number of internal simplifications to JAX, for example in a subsequent change we can probably delete PmapSharding entirely. This in turn also would allow us to delete the APIs `jax.device_put_replicated` and `jax.device_put_sharded`, which predate the current sharding design.
This change also prepares for an upcoming change where we would like to redefine `pmap` in terms of `jit(shard_map(...))`, allowing us to delete most `pmap` code paths.
Once enabled, this change has the potential to break pmap users who:
a) look at the shards of an array, e.g., via `.addressable_shards`, or `jax.make_array_from_single_device_arrays`, since the shapes of the shards will change.
b) rely on zero-copy behavior in APIs like `jax.device_put_replicated`.
The change is disabled by default, so we do not expect any user visible impacts from this change.
PiperOrigin-RevId: 599787818
