[Feature Request] Nested keys compatibility improvement

[vmoens]

Motivation

We need to improve the compatibility with nested keys. This should include:

• TensorDict.get(nested_key)
• TensorDict.set(nested_key, value) (+ creation of the sub-td if the original key is missing)
• TensorDict.set_(nested_key, value)
• TensorDict.keys()
• TensorDict.select(*keys)
• TensorDict.exclude(*keys)
• TensorDict.set_default(key, value)
• TensorDictModule
• TensorDictSequential

We will cover once it's implemented

• TensorDict.pop(key)

Contentious methods / unresolved issues:

• items: we could return the first level by default and return nested ones if a flag is set to true?
• values: same reason
• meta-tensor: building the features in parallel for meta-tensor will double the amount of work (?)
• empty sub-tensordict behaviour: default behaviour would return only leaves, but one could ask for every level OR to return leaf-tensordicts.

cc @tcbegley