fix(fsdp): preserve internal module output dtype (#3657)

* fix(nemotron-v3): align latent projection input dtype

Cast latent MoE expert output at the model-specific projection boundary so nested FSDP FP32 outputs remain compatible with BF16 projection weights.

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

* fix(nemotron-v3): preserve autocast projection dtype

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

* test(nemotron-v3): keep dtype hook compile-safe

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

* fix(fsdp): preserve internal module output dtype

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

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Signed-off-by: HuiyingLi <willwin.lee@gmail.com>