Use CPU PyTorch for testing the SDK (#7825)

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### Motivation and context
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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.

Using the CPU version saves ~4GB of disk space, which is a lot, because
the standard GitHub runners only offer 14 GB.

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Chores**
- Enhanced the installation process by adding an extra index URL for
PyTorch CPU wheels to improve SDK setup reliability.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.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