Cancel in-progress but out-of-date GPU actions (#289)

## Summary
When updating a PR branch, a lot of concurrent CICD actions can be
triggered. This creates a backlog and it takes time for the tests to
actually get around to running on the most recent commit. Github makes
it easy to cancel in-progress actions when new actions are trigged
within a group. This will help avoid wasting GPU time running CICD on
out of date commits.

Avoids cancelations on main, so that a complete CICD history is
available there.

- Hardware Type: <BLANK>
- [x] run `make test` to ensure correctness
- [x] run `make checkstyle` to ensure code style
- [x] run `make test-convergence` to ensure convergence

---------

Co-authored-by: Steven Shimizu <[email protected]>

Author: tyler-romero

.github/workflows/gpu-ci.yml

@@ -3,11 +3,17 @@ name: GPU CI Pipeline
 on:
   push:
     branches:
-      - main 
+      - main
   pull_request:
     branches:
       - main
 
+concurrency:
+  # This causes it to cancel previous in-progress actions on the same PR / branch,
+  # but not on main
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
+
 jobs:
   gpu-ci-tests:
     runs-on: ubuntu-latest