336 ema model weight averaging

[abhaygoudannavar]

Add an Exponential Moving Average (EMA) callback for model weights via a PyTorch Lightning
Callback subclass. EMA maintains a shadow copy of parameters as a running average (θ_ema ← decay * θ_ema + (1 - decay) * θ_current), used for validation/test/inference while training continues on raw optimizer-updated weights. This reduces per-step noise compounding during autoregressive rollouts and produces more stable checkpoints.
Changes:

• neural_lam/callbacks.py (new): EMACallback(pl.Callback) with hooks for weight initialization, in-place lerp_ update, weight swapping during eval, and checkpoint persistence. Handles device transfer correctly when resuming from a checkpoint.
• neural_lam/train_model.py: Added --ema_decay CLI argument (default None = disabled). When set (e.g. 0.999), appends EMACallback to the trainer's callback list.
• tests/test_ema_callback.py(new): 7 tests covering decay validation, mathematical correctness of the running average, weight swap during validation/test, checkpoint save/load round-trip, and no-op behavior when EMA is disabled.
• CHANGELOG.md: Added entry under added.

Motivation:
EMA is standard in every major neural weather prediction system (GraphCast reports 5–12% RMSE reduction over 10-day rollouts). neural-lam is currently the only major open-source neural weather framework without it.

Dependencies:
None beyond existing PyTorch + PyTorch Lightning.

Issue Link #336

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[abhaygoudannavar]

@sadamov i have fixed the issue #336 and i wanted to know if there are any changes that are need to be done.

[sadamov]

@abhaygoudannavar the files changes indicate to solve the pinned memory worker issue and nothing related to EMA. can you double check?

[abhaygoudannavar]

Hey @sadamov! First off—thank you for catching that, you're totally right.
It looks like I accidentally branched off from my earlier pin_memory work instead of a clean
main branch, which is why none of the EMA code actually made it into the diff here.

When I tried to clean up the commit history by resetting the branch directly, GitHub automatically closed this PR. Since this one got pretty messy under the hood, I thought it would be best to just start fresh.

I've opened a clean, new PR with the proper EMA implementation (callbacks, tests, CLI args, etc.) right here: #362

Thanks again for reviewing, and sorry for the extra noise on this one! I'll leave this PR closed and we can continue the review over on the new one.