fix(whisperx): unpin torch versions and use CPU index for cpu requirements

Address review feedback:
- Use --extra-index-url for CPU torch wheels to reduce size
- Remove torch version pins, let uv resolve compatible versions

Signed-off-by: eureka928 <meobius123@gmail.com>

backend/python/whisperx/requirements-cpu.txt

@@ -1,2 +1,3 @@
-torch==2.4.1
+--extra-index-url https://download.pytorch.org/whl/cpu
+torch
 whisperx @ git+https://github.com/m-bain/whisperX.git

backend/python/whisperx/requirements-cublas12.txt

@@ -1,2 +1,2 @@
-torch==2.4.1
+torch
 whisperx @ git+https://github.com/m-bain/whisperX.git

backend/python/whisperx/requirements-cublas13.txt

@@ -1,3 +1,3 @@
 --extra-index-url https://download.pytorch.org/whl/cu130
-torch==2.9.1
+torch
 whisperx @ git+https://github.com/m-bain/whisperX.git