Pin nemo_toolkit to 2.7.3 for parakeet and sortformer

Adding matplotlib as an explicit line in install_requirements.txt
changed pip's resolver behavior: with matplotlib pinned first the
resolver lands on nemo_toolkit-2.0.0 in the sortformer CUDA job, which
predates SortformerEncLabelModel and produces `ImportError: cannot
import name 'SortformerEncLabelModel' from 'nemo.collections.asr.models'`.
The successful 12:51 UTC run on main resolved nemo_toolkit-2.7.3
transitively, with matplotlib coming along for the ride. Pin both
directly so the install order can no longer perturb the resolution.

Authored with Claude Code.

Author: rascani

examples/models/parakeet/install_requirements.txt

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

examples/models/sortformer/install_requirements.txt

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