[CI] run fast gpu tests conditionally on pull requests. (#10310)

* run fast gpu tests conditionally on pull requests.

* revert unneeded changes.

* simplify PR.

.github/workflows/push_tests.yml

 name: Fast GPU Tests on main
 
 on:
+  pull_request:
+    branches: main
+    paths:
+      - "src/diffusers/models/modeling_utils.py"
+      - "src/diffusers/models/model_loading_utils.py"
+      - "src/diffusers/pipelines/pipeline_utils.py"
+      - "src/diffusers/pipeline_loading_utils.py"
   workflow_dispatch:
   push:
     branches:
@@ -160,6 +167,7 @@ jobs:
         path: reports
 
   flax_tpu_tests:
+    if: ${{ github.event_name != 'pull_request' }}
     name: Flax TPU Tests
     runs-on:
       group: gcp-ct5lp-hightpu-8t
@@ -208,6 +216,7 @@ jobs:
         path: reports
 
   onnx_cuda_tests:
+    if: ${{ github.event_name != 'pull_request' }}
     name: ONNX CUDA Tests
     runs-on:
       group: aws-g4dn-2xlarge
@@ -256,6 +265,7 @@ jobs:
         path: reports
 
   run_torch_compile_tests:
+    if: ${{ github.event_name != 'pull_request' }}
     name: PyTorch Compile CUDA tests
 
     runs-on:
@@ -299,6 +309,7 @@ jobs:
         path: reports
 
   run_xformers_tests:
+    if: ${{ github.event_name != 'pull_request' }}
     name: PyTorch xformers CUDA tests
 
     runs-on:
@@ -349,7 +360,6 @@ jobs:
     container:
       image: diffusers/diffusers-pytorch-cuda
       options: --gpus 0 --shm-size "16gb" --ipc host
-
     steps:
     - name: Checkout diffusers
       uses: actions/checkout@v3
@@ -359,7 +369,6 @@ jobs:
     - name: NVIDIA-SMI
       run: |
         nvidia-smi
-
     - name: Install dependencies
       run: |
         python -m venv /opt/venv && export PATH="/opt/venv/bin:$PATH"