Migrate experimental 3rd party dependencies inside repo

[theOGognf]

Some dependencies we use are very useful but are still in their early stages and can be subject to breaking changes. It's probably better to manually move into this project as a submodule for now rather than adding them to a requirement. We can easily move back to using their wheels directly once they become more stable. The main ones include:

• torchrl's tensor specs
• tensordict
• dadaptation optimizers

[theOGognf]

This was completed with 050b087 with the exception of tensordict. That one is a bit more complex to add independent of the actual package