Fix broken fully_shard reference in FullyShardedModule docstring - #4007

Proposed changes

FullyShardedModule's docstring says "Use :func:fully_shard to wrap a module", but that link is dead:

python/mlx/nn/layers/distributed.py:docstring of
mlx.nn.layers.distributed.FullyShardedModule:11:
WARNING: py:func reference target not found: fully_shard [ref.func]

The two are documented under different module contexts in nn/distributed.rst. fully_shard sits under .. currentmodule:: mlx.nn.layers.distributed, while FullyShardedModule is under .. currentmodule:: mlx.nn, so on that page the bare name resolves against mlx.nn and finds nothing.

Qualified it, using the ~ form that the same file already uses for :func:~mlx.core.quantize``, so the rendered text stays fully_shard. After the change the reference resolves.

One thing worth flagging: I noticed the commit that added this landed as "[WIP] [CUDA] fsdp" (#3768). I still thought this was worth sending because both fully_shard and FullyShardedModule are already listed in nn/distributed.rst, so this only repairs a link between two pages you have already made public rather than exposing anything new. If you would rather leave the fsdp docs untouched until that work settles, please just close this, no problem at all.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

(one line, docstring text only; verified by building the docs before and after)

context on me: freshman contributor, i use Claude Code to help sweep for this kind of thing. you closed my gather_qqmm docs PR earlier today because that API is still in progress, which was a fair call and taught me to check whether something is meant to be public before touching it. that is why i checked here that both names are already in the docs, and why i am flagging the WIP origin rather than quietly shipping it.