Fix sa3-m DiT ONNX external_data ref (was pointing at wrong filename)

The medium DiT FP16-mixed ONNX had 428 initializers whose external_data
location field pointed at "dit_m_fp16mixed.onnx.data", but the actual
sidecar on disk (and in this repo) is "dit_fp16mixed.onnx.data" — no
"_m_" infix. The mismatch came from the producer naming convention at
build time; the rename to the shipped name only touched the file, not
the in-ONNX reference.

Consequence: any consumer trying to compile a TRT engine from this ONNX
(e.g., on a new GPU arch like sm_120 via build_from_onnx.py sa3-m) hit:

[TRT] [E] WeightsContext.cpp:190: Failed to open file: ...
/onnx/sa3-m/dit_m_fp16mixed.onnx.data
[TRT] [E] Failed to import initializer: padding_embedding

… and ended up either with no engine or a partial/garbage engine,
manifesting as "medium DiT outputs ~5s of music then dead silence" on
some setups. The pre-built sm_90 TRT engine in this repo is unaffected
(it was built from the producer's ONNX at the moment the data file still
had the original name).

Fix: walked all 428 initializers and rewrote the external_data location
to "dit_fp16mixed.onnx.data". The .data file is unchanged.

Verified: TRT 10.15 OnnxParser parses the fixed ONNX cleanly (44909
layers, 6 inputs, 1 output).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

onnx/sa3-m/dit_fp16mixed.onnx

@@ -1,3 +1,3 @@
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-size 4249742
+oid sha256:ccc51b4e067ad1132b84be71f11797da4eae2b3544977bcd52466d23e7a824d7
+size 4248874