Removing _CustomOpTensorDict classes

[vmoens]

_CustomOpTensorDict is an abstract class for some operations (mainly on TensorDict shapes).
It may not be necessary to keep it and maintaining it may be bothersome in the future.

Pros:

• Most operations that we'd be doing would not change the storage anyway in most cases. For instance, a call on the view method of a regular TensorDict's values will return a view on those tensors, with no extra memory cost.
• Code clarity: this is one more class that inherits from _TensorDict, and each new operation will (in the current implementation) require its own class. This is not ideal.

Cons:

• In the case of LazyStackedTensorDict, we would probably like to have a view method that keeps the storage location unchanged. Not using _CustomOpTensorDict will require us to call contiguous on the stack and then call the view operation, but this will assign a new storage to the resulting tensors. The final option could be to prevent users from calling these fixed-storage operations on LazyStackedTensorDict but that could lead to some issues in the future (i.e. users asking for this feature when it has been decided to deprecate it previously).
• The ProbabilisticOperator class can write to tensordict instances that are unsqueezed. This makes it easy to do in-place modifications of a tensordict without having to create multiple copies of the tensors. This feature is used to execute a policy on a single environment that has no batch size.

[vmoens]

Closing as _CustomOpTensorDict is required by a bunch of current components in the code.
Happy to re-open if needed