[BUG] `load_state_dict` fails if checkpoint lacks entries for TensorDictParams

[egaznep]

The context is rather hard to explain, but I have already identified the problem.

Culprit:

tensordict/tensordict/nn/params.py

Lines 1244 to 1247 in f7b5e05

	prefix = tuple(key for key in prefix.split(".") if key)
	if prefix:
	data = data.get(prefix)
	self.data.load_state_dict(data)

If prefix is not present in data, L1246 would set data = None and the subsequent line fails. I guess the following codeblock would solve it?

        if prefix:
            data = data.get(prefix)
        if data is not None:
            self.data.load_state_dict(data)