Improve CI build speed by letting bazel decide how many worker threads to have (#9209)

Co-authored-by: Zhanyong Wan <[email protected]>

Author: zhanyong-wan

.github/workflows/_build_plugin.yml

@@ -33,7 +33,7 @@ jobs:
       image: ${{ inputs.dev-image }}
     env:
       GCLOUD_SERVICE_KEY: ${{ secrets.gcloud-service-key }}
-      BAZEL_JOBS: 16
+      BAZEL_JOBS: ''  # Let bazel decide the parallelism based on the number of CPUs.
       BAZEL_REMOTE_CACHE: ${{ github.event_name == 'push' || github.event.pull_request.head.repo.full_name == github.repository }}
     steps:
       - name: Checkout actions

.github/workflows/_build_torch_xla.yml

@@ -38,7 +38,7 @@ jobs:
     env:
       GCLOUD_SERVICE_KEY: ${{ secrets.gcloud-service-key }}
       BAZEL_REMOTE_CACHE: ${{ github.event_name == 'push' || github.event.pull_request.head.repo.full_name == github.repository }}
-      BAZEL_JOBS: 16
+      BAZEL_JOBS: ''  # Let bazel decide the parallelism based on the number of CPUs.
       BUILD_CPP_TESTS: 1
     steps:
       # Need to check out local composite actions before using them

.github/workflows/_test.yml

@@ -78,7 +78,7 @@ jobs:
       RUN_TORCH_MP_OP_TESTS: ${{ matrix.run_torch_mp_op_tests }}
       RUN_CPP_TESTS1: ${{ matrix.run_cpp_tests1 }}
       RUN_CPP_TESTS2: ${{ matrix.run_cpp_tests2 }}
-      BAZEL_JOBS: 16
+      BAZEL_JOBS: ''  # Let bazel decide the parallelism based on the number of CPUs.
       BAZEL_REMOTE_CACHE: 1
     steps:
       - name: Checkout actions

.github/workflows/_test_requiring_torch_cuda.yml

@@ -48,7 +48,7 @@ jobs:
     timeout-minutes: ${{ inputs.timeout-minutes }}
     env:
       USE_COVERAGE: ${{ inputs.collect-coverage && '1' || '0' }}
-      BAZEL_JOBS: 16
+      BAZEL_JOBS: ''  # Let bazel decide the parallelism based on the number of CPUs.
       BAZEL_REMOTE_CACHE: 1
     steps:
       - name: Checkout actions

.github/workflows/build_upstream_image.yml

@@ -14,7 +14,7 @@ jobs:
     timeout-minutes: 30
     env:
       ECR_DOCKER_IMAGE_BASE: 308535385114.dkr.ecr.us-east-1.amazonaws.com/pytorch/xla_base
-      BAZEL_JOBS: 16
+      BAZEL_JOBS: ''  # Let bazel decide the parallelism based on the number of CPUs.
     steps:
       # See https://github.com/actions/checkout/issues/1014#issuecomment-1906802802
       - name: Clean up workspace