[Feature Request] More arguments for td.memmap_()

[vmoens]

Motivation

Since TensorDict.memmap_() creates a copy on physical mem of the content of a tensordict, it would be nice to have some more options:

• MemmapTensors are temporary files. It could be nice to disable this when calling memmap_() (like temporary=False). In that way, one would know that the content of the memmap will be kept in memory after the process exits.
• We should also be able to pass the path to where the files have to be saved (`path="/path/to/my_tensordict"). This would work nicely with [Feature Request] Saving and loading memmap tensordicts / tensorclasses #176
• Similarly, it could be neat to be able to control if a memmap_ tensor can be written or not (by passing mode="r+" for instance). Of course, memmap_() would always write the content but then modify the permissions accordingly to make sure that the content is somewhat locked for in-place modifs.

[vmoens]

cc @sreevasthav