Change GenAI OSS runner to fix OOM

Summary:
Switch to runners with larger memory runner to fix OOM for GenAI OSS CI.

The GenAI OSS CI build jobs fail with

```
The self-hosted runner lost communication with the server. Verify the machine is running and has a healthy network connection. Anything in your workflow that terminates the runner process, starves it for CPU/Memory, or blocks its network access can cause this error.
```

The runners for CUDA12.8 for python 3.10 3.12 3.13 consistently failed.
For example:
https://github.com/pytorch/FBGEMM/actions/runs/14731902981/job/41348012076
https://github.com/pytorch/FBGEMM/actions/runs/14727090721/job/41332229097
https://github.com/pytorch/FBGEMM/actions/runs/14722058017/job/41317562384

Cause from huydhn:
- The error happens when the runner runs out of memory.
- This is a common bottleneck for the build job.
- linux.24.large probably spawns too many processes given its higher number of CPU cores and OOM

Reviewed By: huydhn

Differential Revision: D74221851

fbshipit-source-id: 71ce45cd0e204ee8055cec154a02ae53fb581c96

Author: spcyppt

.github/workflows/fbgemm_gpu_ci_genai.yml

@@ -70,7 +70,7 @@ jobs:
       fail-fast: false
       matrix:
         host-machine: [
-          { arch: x86, instance: "linux.24xlarge" },
+          { arch: x86, instance: "linux.8xlarge.memory" },
         ]
         python-version: [ "3.9", "3.10", "3.11", "3.12", "3.13" ]
         cuda-version: [ "11.8.0", "12.6.3", "12.8.0" ]

.github/workflows/fbgemm_gpu_release_genai.yml

@@ -69,7 +69,7 @@ jobs:
       fail-fast: false
       matrix:
         host-machine: [
-          { arch: x86, instance: "linux.24xlarge" },
+          { arch: x86, instance: "linux.12xlarge.memory" },
         ]
         python-version: [ "3.9", "3.10", "3.11", "3.12", "3.13" ]
         cuda-version: [ "11.8.0", "12.6.3", "12.8.0" ]