fix(batching): stop shorter clips in a batch from failing to transcribe

[richiejp]

Basically on streaming models with a short attention window the padding
that is applied to the shorter clips can cause a scascade of Infs/Nans
that causes transcription to fail and return empty.

---

In a heterogeneous-length batch on a chunked_limited / local-attention
model (e.g. nemotron-3.5-asr-streaming-0.6b), any clip whose padding tail
exceeded the attention left context (~4.5 s of length spread at 8x
subsampling) decoded to an EMPTY transcript. Mechanism: a deeply padded
query row has its ENTIRE additive mask row at -inf, softmax over all--inf
yields NaN, the post-softmax query-row mask keeps it (NaN*0 == NaN), and
the NaN key columns then poison every softmax row of the next layer --
24 layers later the whole item is NaN. Full attention was unaffected
(padded query rows can still see valid keys), which is why only
limited-context models in batches with enough length spread were hit; the
batch JSON entry points the LocalAI backend calls were silently affected.

Leave fully-masked padded query rows unmasked instead: their finite
garbage output is zeroed by the existing post-softmax query mask (output
reduces to linear_out.bias, exactly as before), and valid rows' masks are
untouched. Applied to every band/window mask builder (single + batched,
full/chunked-limited + banded local + chunked kernel).

Verified on nemotron-3.5-asr-streaming-0.6b q8_0 (att_context [56,3]): in
a {16.4 s, 7.4 s} batch the short item's encoder output was 100% NaN
before the fix and now byte-matches its solo decode; transcripts match
solo at every tested T_max spread (8.4-16.4 s). Also verified on
parakeet_realtime_eou_120m-v1 f16 (att_context [70,1]): pre-fix the 7.4 s
fixture decoded empty in any batch with T_max >= 14.4 s, post-fix it
byte-matches solo at every spread. Cache-aware streaming and single-clip
offline transcripts are byte-identical between pre-fix and post-fix
builds (those paths never form padded batches), so the fix changes
nothing outside the broken case.

Assisted-by: Claude:claude-fable-5 [Claude Code]