Use CPU PyTorch for testing the SDK

Not only is the GPU version of PyTorch much bigger than the CPU version,
but it also pulls in CUDA, which is enormous. We don't (and can't) use any
GPU features in our tests, so we don't need the GPU version.

Eliminating it saves ~4GB of disk space, which is precious when it comes to
the GitHub actions runners.

.github/workflows/full.yml

@@ -160,7 +160,8 @@ jobs:
       - name: Install SDK
         run: |
           pip3 install -r ./tests/python/requirements.txt \
-            -e './cvat-sdk[pytorch]' -e ./cvat-cli
+            -e './cvat-sdk[pytorch]' -e ./cvat-cli \
+            --extra-index-url https://download.pytorch.org/whl/cpu
 
       - name: Running REST API and SDK tests
         id: run_tests

.github/workflows/main.yml

@@ -170,7 +170,8 @@ jobs:
       - name: Install SDK
         run: |
           pip3 install -r ./tests/python/requirements.txt \
-            -e './cvat-sdk[pytorch]' -e ./cvat-cli
+            -e './cvat-sdk[pytorch]' -e ./cvat-cli \
+            --extra-index-url https://download.pytorch.org/whl/cpu
 
       - name: Run REST API and SDK tests
         id: run_tests