[Feature] Truly invertible tensordict permutation of dimensions

[ramonmedel]

Description

Inverse operations on Tensordicts can return the original data. This is currently not the case for tensordict.permute.

We need to return the original tensordict when calling tensordict.permute(a, b, c) we get an instance of a PermutedTensorDict and if we call again the same method with the same parameters in the resulting PermutedTensorDict

assert tensordict.permute(dims_list, dim).permute(dims_list, dim) is tensordict

Motivation and Context

close #278

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[vmoens]

Can you use something like the test function i put in the comment instead?
Otherwise it looks great thanks for this!

[vmoens]

LGTM thanks for contributing!