[Feature] Feat/diffusion bc loss

[theap06]

[Feature] Add DiffusionBCLoss objective and Pendulum BC example" --body

Summary

Fixes #3149

• Implements DiffusionBCLoss in torchrl/objectives/diffusion_bc.py — a LossModule subclass that computes the ε-prediction (noise-prediction) denoising loss from Diffusion Policy (Chi et al., RSS 2023)
• Completes Phase-1 of the diffusion policy feature alongside DiffusionActor ([Feature] Diffusion Actor DDPMModule #3596)
• Adds 17 unit tests and an end-to-end Pendulum-v1 BC example

Design

The loss:

1. Samples a random timestep t per batch element
2. Corrupts the clean demonstration action via _DDPMModule.add_noise(clean_action, t) (forward diffusion)
3. Runs the score network on (noisy_action || observation || t)
4. Returns MSE between predicted noise and actual noise as loss_diffusion_bc

Supports set_keys() for observation/action key remapping and configurable reduction.

Files changed

File	Description
torchrl/objectives/diffusion_bc.py	DiffusionBCLoss implementation
torchrl/objectives/__init__.py	Register DiffusionBCLoss
test/objectives/test_diffusion_bc.py	17 tests (output keys, backward, gradient flow, custom keys, convergence)
examples/diffusion_bc_pendulum.py	End-to-end BC training on Pendulum-v1

Test plan

• pytest test/objectives/test_diffusion_bc.py — 17/17 passing
• pre-commit run — all hooks passing
• Forward + backward smoke tested locally

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/3604

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⚠️ 17 Awaiting Approval

As of commit ef2554c with merge base f54a7c7 ():

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