Refactor `CompositeSpec` tests

[vmoens]

CompositeSpec tests need improvements:

Test all CompositeSpec features

Right now we only have a single function to test all of the CompositeSpec methods. This should be chunked in several pieces:

class TestComposite:
    @pytest.mark.parametrize("shape", [[], [3]])
    @pytest.mark.parametrize("device", get_available_devices())
    def test_composite_rand(device, shape):
        do_stuff()

    def test_composite_encode(...):
        do_stuff()

These tests should cover all the available devices.
The methods to test are:

• to (device casting): test that an exception is raised if one inputs a dtype. Test that the device corresponds to the device used for casting. Test that random values drawn after casting to the device are on the device.
• __getitem__ and __setitem__
• del_
• encode: inputting a dictionary should return the same dictionary modified in-place with the corresponding encodings.
• __repr__ should return a readable string
• type_check should work with either a tensor and a key OR with a dict/TensorDict.
• is_in
• project
• rand

Test CompositeSpec with incomplete spec fields

CompositeSpec can have None values when we know that a value is expected but we don't need to bother defining its space.
The current tests for CompositeSpec only cover the case where all the fields are defined.
We should define a second round of tests where one of the values is None.
This test should somewhat look like

@pytest.mark.parametrize("incomplete", [True, False])
def test_composite(incomplete, ...):
    do_stuff()