[Feature Request] Write tests for functorch integration

functorch does not play well with TensorDictModules (especially when callingvmap).
Therefore we need to functionalize the modules inside the TensorDictModule, and then build some interface between the TensorDictModule and the functional module.
For instance, TDSequence has to split the parameters according to each of the submodules etc.

We should test all of these functionalities:

• creating FunctionalModuleWithBuffers from the TDModule OR providing FunctionalModuleWithBuffers to a TDModule upon construction
• Nested TDModules (e.g. ProbabilisticTensorDictModule): param length, casting etc.
• TDSequence param split
• [upcoming feature] TDSequence param split: caching of param lengths
• FunctionalModule and FunctionalModuleWithBuffers (some logics might break with the former)
• vmap extension given number of input to module