Drop stale param refs in _update_param_with_optimizer to cut VRAM spike

[Evggamgam]

_update_param_with_optimizer keeps the old Parameter alive via three references (local var, optimizer.state key, param_groups["params"]) while new Adam state tensors are allocated, causing a per-densify transient of ~1× size(largest param) above steady state. Rebind those refs to new_param and del the local before optimizer_fn, so the old tensor is freed during the state reshape.

Pure reordering — output Parameter and Adam state are bit-identical to the pre-patch implementation; wall time unchanged.

Max Gaussians in the densify-step budget on a 24 GB card (excluding render/backward overhead):

strategy	before	after
DefaultStrategy	16.5M	19.8M
MCMCStrategy	21.3M	25.4M