[ci/docs] add some documentation about the stats uploading process

This process is pretty confusing, so wrote it down.

[skip ci]

Pull Request resolved: pytorch#79504

Approved by: https://github.com/janeyx99

Author: suo

tools/stats/README.md

@@ -0,0 +1,31 @@
+# PyTorch CI Stats
+
+We track various stats about each CI job.
+
+1. Jobs upload their artifacts to an intermediate data store (either GitHub
+   Actions artifacts or S3, depending on what permissions the job has). Example:
+   https://github.com/pytorch/pytorch/blob/a9f6a35a33308f3be2413cc5c866baec5cfe3ba1/.github/workflows/_linux-build.yml#L144-L151
+2. When a workflow completes, a `workflow_run` event [triggers
+   `upload-test-stats.yml`](https://github.com/pytorch/pytorch/blob/d9fca126fca7d7780ae44170d30bda901f4fe35e/.github/workflows/upload-test-stats.yml#L4).
+3. `upload-test-stats` downloads the raw stats from the intermediate data store
+   and uploads them as JSON to Rockset, our metrics backend.
+
+```mermaid
+graph LR
+    J1[Job with AWS creds<br>e.g. linux, win] --raw stats--> S3[(AWS S3)]
+    J2[Job w/o AWS creds<br>e.g. mac] --raw stats--> GHA[(GH artifacts)]
+
+    S3 --> uts[upload-test-stats.yml]
+    GHA --> uts
+
+    uts --json--> R[(Rockset)]
+```
+
+Why this weird indirection? Because writing to Rockset requires special
+permissions which, for security reasons, we do not want to give to pull request
+CI. Instead, we implemented GitHub's [recommended
+pattern](https://securitylab.github.com/research/github-actions-preventing-pwn-requests/)
+for cases like this.
+
+For more details about what stats we export, check out
+[`upload-test-stats.yml`](https://github.com/pytorch/pytorch/blob/d9fca126fca7d7780ae44170d30bda901f4fe35e/.github/workflows/upload-test-stats.yml)