fix: clean up detailed history for CHANGELOG (#1494)

# What does this PR do?

- do not dump all commit history in CHANGELOG
cc @terrytangyuan 

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
```
python scripts/gen-changelog.py
```

[//]: # (## Documentation)

scripts/gen-changelog.py

@@ -4,38 +4,71 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-import requests
 import os
 
+import requests
+
+
 def get_all_releases(token):
     url = f"https://api.github.com/repos/meta-llama/llama-stack/releases"
     headers = {"Accept": "application/vnd.github.v3+json"}
-    
+
     if token:
         headers["Authorization"] = f"token {token}"
-    
+
     response = requests.get(url, headers=headers)
-    
+
     if response.status_code == 200:
         return response.json()
     else:
-        raise Exception(f"Error fetching releases: {response.status_code}, {response.text}")
+        raise Exception(
+            f"Error fetching releases: {response.status_code}, {response.text}"
+        )
+
+
+def clean_release_body(body):
+    """Remove '## All changes' sections from release notes."""
+    lines = body.split("\n")
+    cleaned_lines = []
+    skip_mode = False
+
+    for line in lines:
+        if line.strip() in [
+            "## All changes",
+            "### What's Changed",
+            "## What's Changed",
+            "## New Contributors",
+        ]:
+            skip_mode = True
+        elif skip_mode and line.startswith("##"):
+            # Found a new section, stop skipping
+            skip_mode = False
+            cleaned_lines.append(line)
+        elif not skip_mode:
+            cleaned_lines.append(line)
+
+    return "\n".join(cleaned_lines)
 
 
 def merge_release_notes(output_file, token=None):
     releases = get_all_releases(token)
-    
+
     with open(output_file, "w", encoding="utf-8") as md_file:
         md_file.write(f"# Changelog\n\n")
-        
+
         for release in releases:
             md_file.write(f"# {release['tag_name']}\n")
             md_file.write(f"Published on: {release['published_at']}\n\n")
-            md_file.write(f"{release['body']}\n\n")
+
+            # Clean the release body to remove "## All changes" sections
+            cleaned_body = clean_release_body(release["body"])
+            md_file.write(f"{cleaned_body}\n\n")
+
             md_file.write("---\n\n")
-    
+
     print(f"Merged release notes saved to {output_file}")
 
+
 if __name__ == "__main__":
     OUTPUT_FILE = "CHANGELOG.md"
     TOKEN = os.getenv("GITHUB_TOKEN")