perf: make MSE observer compatible with torch.compile

- Replace patch_attr context manager with pre-created token_args
- Replace data-dependent control flow (torch.any + early stopping) with torch.where
- MSE observer now passes torch.compile fullgraph=True
- Benchmark: 39x speedup with inductor backend

Partial fix for #1485

Signed-off-by: Jaewoo Kim <pewpewplay315@gmail.com>

Author: Bias92