[Feature Request] Test TensorDict representation

[vmoens]

Motivation

Some misrepresentations of tensordict have been reported (e.g. #367)
We should test the following types:

• plain TensorDict with regular tensors
• nested tensordicts
• stacked tensordict (nested)
• indexed tensordicts (nested):
  • with integers
  • with slice
  • with None
  • with tensors
  • with masks
  • indexed with integer from stacked
• when moved from device to device
• when modifying the batch_size

The test should write explicitely what the repr string is and match it against the __repr__ output.
When writing the test, one should make sure that the print output matches the tensordict content.