feat: nemotron 3.5 cfg (#3442)

* feat(recipes): add nemotron MTP PEFT finetune example

Adds a LoRA finetuning example for Nemotron hybrid mamba-attention MoE
checkpoints with multi-token prediction, using 2 weight-tied MTP
iterations of a single physical MTP layer (mirrors Megatron's
--mtp-use-repeated-layer).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: adil-a <adil.asif2000@hotmail.com>

* feat(recipes): use TE backends with gmm experts in nemotron MTP recipe

Switch the backend block to attn/linear/rms_norm=te with gmm experts and
the hybridep dispatcher (matching the ultra recipe), drop the inert
torch.compile block, and raise ep_size to 8.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: adil-a <adil.asif2000@hotmail.com>

* feat(recipes): switch nemotron MTP recipe dispatcher to deepep

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: adil-a <adil.asif2000@hotmail.com>

* fix(nemotron_v3): report view-loaded MTP expert keys under mtp. prefix

The MTP pass of from_hf records in-place (strided-view) loaded expert keys
in mtp.-stripped form because the shared merge loop only sees stripped
keys. The checkpoint loader diffs view_loaded_native_keys against the
model's real parameter names, so the stripped entries surfaced as a false
'Checkpoint key mismatch' warning (missing mtp.layers.N.mixer.experts.*,
unexpected layers.N.mixer.experts.*) on every MTP checkpoint load — and
would raise spuriously under safe custom-MoE TP. Re-prefix the entries the
MTP merge adds so the key-diff matches them. Weights were always loaded
correctly; this fixes only the bookkeeping.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: adil-a <adil.asif2000@hotmail.com>

* feat(recipes): name Nemotron Nano v3.5 Lightning in MTP PEFT recipe

Rename the redacted nemotron MTP recipe to
nemotron_nano_v3_5_lightning_hellaswag_peft.yaml and point it at the
public nvidia/NVIDIA-Nemotron-3.5-Lightning-30B-A3B-BF16 checkpoint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: adil-a <adil.asif2000@hotmail.com>

* docs(recipes): drop Megatron flag mention from lightning recipe comment

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: adil-a <adil.asif2000@hotmail.com>

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Signed-off-by: adil-a <adil.asif2000@hotmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>