add conditional requirements for pytorch on darwin

Author: Abe404

trainer/requirements.txt

@@ -5,6 +5,12 @@ pytest==8.4.2
 scikit-image==0.25.2
 scipy==1.16.2
 pillow==12.0.0
-torch==2.6.0+cu124
-torchvision==0.21.0+cu124
 
+# ---- PyTorch: platform-specific pins ----
+# macOS (MPS/CPU wheels from PyPI)
+torch==2.6.0; sys_platform == "darwin"
+torchvision==0.21.0; sys_platform == "darwin"
+
+# Windows/Linux CUDA 12.4
+torch==2.6.0+cu124; sys_platform != "darwin"
+torchvision==0.21.0+cu124; sys_platform != "darwin"