fix(loss): min/max packed extrema instead of summing them - #3901

SequencePackingLossWrapper folds per-sequence metric dicts into one. It
special-cases extrema through a hardcoded allowlist naming only the four
probs_ratio keys, so MseValueLossFn's values_min/values_max fall through
to '+=' and are summed. Three packed sequences spanning -3..9 report
values_min=4.0 -- a positive number for a critic whose predictions go
negative -- and values_max=15.0. The error grows with packing density,
so it is not a stable offset a reader could correct for.

The loss function already says what these are: 'Min/max are per-MB;
ppo.py takes min/max across MBs.' Five other sites apply the
'_min'/'_max' suffix rule to this very dict, and one of them --
megatron_value_worker.py:611 -- is this wrapper's own direct consumer,
skipping the divide because it is an extremum while the wrapper upstream
has already summed it. Use the same rule here.

That alone is not enough. MseValueLossFn returns 0.0 for a fully-masked
sequence where ClippedPGLossFn returns +/-inf, and 0.0 is a plausible
value that wins the min against an all-positive critic: the one-line
version reports 0.0 where the truth is 3.0. sample_mask is
loss_multiplier, which overlong_filtering zeroes per sample, and under
packing one filtered sample in a pack is enough. So the sentinel moves
to +/-inf and ppo.py skips it, matching what that file already does for
probs_ratio at :1774 and :2757.

Metrics only -- packed and unpacked losses are bit-identical. Reachable
on ppo-qwen2.5-1.5b-gsm8k-1n8g-megatron-valuetp2sp-pp2cp2-pack, which
nightly.txt runs.

Signed-off-by: Tianyi Zhang <123608656+tianyi-zhang-02@users.noreply.github.com>