Add explicit matplotlib dep for parakeet and sortformer (pytorch#19231)

### Summary
The Metal, MLX, and CUDA Windows export workflows started failing on
2026-04-30 with `ModuleNotFoundError: No module named 'matplotlib'` when
the parakeet/sortformer export scripts import `nemo.collections.asr`.
NeMo's clustering_diarizer module top-level imports matplotlib via
vad_utils, but matplotlib is not declared by `nemo_toolkit[asr]`. A
release in some transitive dep stopped pulling matplotlib in
incidentally. nemo_toolkit itself was unchanged (2.7.3 since
2026-04-23), so adding matplotlib explicitly is the targeted fix.

Authored with Claude Code.

### Test plan
CI

cc @metascroy

Author: rascani

examples/models/parakeet/install_requirements.txt

@@ -1 +1,2 @@
-nemo_toolkit[asr]
+nemo_toolkit[asr]==2.7.3
+matplotlib

examples/models/sortformer/install_requirements.txt

@@ -1 +1,2 @@
-nemo_toolkit[asr]
+nemo_toolkit[asr]==2.7.3
+matplotlib