Remove dead PyTorch < 2.6 version guards (#21870)
* Remove dead PyTorch < 2.6 version guards
The torch floor was raised to 2.6 in #21851, so the
_TORCH_GREATER_EQUAL_2_2 .. _TORCH_GREATER_EQUAL_2_5 guards (plus the
unused _TORCH_EQUAL_2_4_0 / _TORCH_GREATER_EQUAL_2_4_1 constants) are
always true and their else-branches unreachable. Drop the constants,
inline the surviving branches, and remove the tests that only covered
the now-impossible paths.
* Add CHANGELOG entries for the removed torch version guards
* Fix tests referencing symbols dropped from the strategy modules
- import the DCP reader/writer helpers from lightning.fabric.utilities.cloud_io,
where they are defined, instead of the FSDP re-export
- stop patching model_parallel._lazy_load, which the module no longer uses;
the remote-vs-mmap regression test now watches torch.load instead
* Address review: drop kwargs indirection and stale comment
The `kwargs` dict in the six `_setup_distributed` implementations only
existed to build the argument list conditionally. Now that both entries
are unconditional, pass them directly as keyword arguments.
Also drop the `_lazy_load` mention from the model_parallel comment; that
path uses `torch.load(mmap=True)` only.
