[BugFix] Preserve lazy cat for mixed TensorDict inputs

[vmoens]

Summary

• route mixed lazy/plain TensorDict cat through lazy cat normalization
• convert dense inputs to compatible lazy stacks instead of densifying lazy stacks
• add coverage for stack-dim, non-stack-dim, non-densifiable, zero-length, and dense out cases

Tests

• uv run --extra tests python -m pytest test/tensordict/test_lazy.py -q