Add recursion guard and tidy tests

[tcbegley]

Description

This PR adds a new decorator _recursion_guard which catches recursion errors in methods that don't currently support auto-nesting and adds a more instructive error message.

I've removed skips from a number of methods that are in fact working. I've also removed some skips from tests which were failing for reasons that should be fixed rather than because they were testing for unsupported behaviour. Reasons for failures are listed below. I did this because otherwise I think we could trick ourselves into thinking that #201 is ready to be merged into main prematurely.

Reasons for failing tests

The following tests fail because assert_allclose_td does not support auto-nested values

• test_from_empty
• test_masking
• test_getitem_ellipsis
• test_getitem_range

The following tests fail because we can't instantiate a TensorDict from a Python dict with auto-nested values

• test_broadcast
• test_equal_dict
• test_nested_dict_init

Finally test_nestedtensor_stack is failing because LazyStackedTensorDict.contiguous is broken. I think a few other methods for LazyStackedTensorDict could be broken but not caught by the tests. The issues here largely stem from the fact that values are computed lazily, and hence use of id to check for repeated values is brittle.