Create dependabot rule to upgrade TorchFix version (#3208)

Summary:
The parameters are from From https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file

### Testing
On my own fork https://github.com/huydhn/executorch/blob/main/.github/dependabot.yml, and the PR to upgrade TorchFix is created successfully huydhn#2

Pull Request resolved: #3208

Reviewed By: kit1980

Differential Revision: D56428297

Pulled By: huydhn

fbshipit-source-id: 8b4f9d638d208fe6f476efdf7667058b2d2ae2fc

Author: huydhn

.github/dependabot.yml

@@ -0,0 +1,11 @@
+# From https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    reviewers:
+      - "pytorch/team-executorch"
+    allow:
+      - dependency-name: "torchfix"