[BugFix] Resolve CompositeDistribution name_map with unraveled keys

[YeonwooSung]

Problem

CompositeDistribution.__init__ and from_distributions unraveled both sides of name_map, then looked the distribution up with the original key:

write_name = unravel_key(name_map.get(name, name_unravel))

dict.get never raises, so the except KeyError was dead. A tuple key does not survive unravel_key (("a",) becomes "a", and (("nested",), "disc") becomes ("nested", "disc")), so the lookup missed and the sample kept the unraveled distribution name. String keys only worked because unravel_key("a") is still "a". A name_map entry that did not match any distribution was ignored, despite from_distributions documenting that case as a KeyError.

Fix

Both constructors resolve name_map through one helper. Distribution names are matched after unravel_key, and the sample is written under the mapped key. Names omitted from name_map keep their own name. A name_map key that does not match any distribution raises KeyError.

Tests

TestCompositeDist covers tuple-key rename, nested tuple-key rename, string-key rename, and an unknown name_map key, for __init__ and from_distributions.

Before the fix, the tuple-key tests failed with the sample key left as "a" (nested keys stayed ("nested", "disc")), and the unknown-key tests did not raise. String keys already passed.

After the fix:

pytest test/nn/test_nn.py::TestCompositeDist -q --tb=short
85 passed

ruff check and ruff format are clean on tensordict/nn/distributions/composite.py and test/nn/test_nn.py.