Feature/add multitask diffusion transformer policy implementation

[brysonjones]

What this does

This PR adds support for an implementation of Multitask Diffusion Transformer Policy, which was shown in a demo of Boston Dynamics' Atlas robot performing whole-body manipulation tasks

I wanted to dive into the research of this method, and build an open-source implementation for the community to leverage and build from.

I will simultaneously be releasing a blog post that includes the details of this work as this gets merged in! (Note: the current blog post link is broken until being released)

How it was tested

• I added a simple test script test_multi_task_dit_policy.py to validate any install and import errors
• I've tested launching training runs on Modal with batch sizes of 320 on H200 and B200 GPUs
• I've trained multiple policies for >60k training steps and >20 hours with this policy, and deployed them onto a Trossen WidowAI Arm (for which I have another PR open for to integrate)

How to checkout & try? (for the reviewer)

Run the test script:

uv run pytest tests/policies/test_multi_task_dit_policy.py -v

Train a policy:

lerobot-train \
  --policy.type=multi_task_dit \
  --dataset.repo_id={{your_dataset_name}} \
  --dataset.root={{your/dataset/path}} \
  --output_dir=outputs/train/multi_task_dit \
  --job_name=multi_task_dit_training_test \
  --policy.device=cuda \
  --batch_size=16 \
  --steps=10000 \
  --save_freq=1000 \
  --wandb.enable=true \
  --policy.repo_id=YOUR_HF_USERNAME/multi_task_dit_policy_test