Add a verify-download command to llama CLI (#457)

# What does this PR do?

It is important to verify large checkpoints downloaded via `llama model
download` because subtle corruptions can easily happen with large file
system writes. This PR adds a `verify-download` subcommand. Note that
verification itself is a very time consuming process (and will take
several **minutes** for the 405B model), hence this is a separate
subcommand (and not part of the download which can already be
time-consuming) and there are spinners and a bit of a "show" around it
in the implementation.

## Test Plan

<img width="1012" alt="image"
src="https://github.com/user-attachments/assets/f82b0d42-2a15-4917-b85e-6d3cd7d31e55">

llama_stack/cli/verify_download.py

@@ -0,0 +1,144 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import argparse
+import hashlib
+from dataclasses import dataclass
+from functools import partial
+from pathlib import Path
+from typing import Dict, List, Optional
+
+from rich.console import Console
+from rich.progress import Progress, SpinnerColumn, TextColumn
+
+from llama_stack.cli.subcommand import Subcommand
+
+
+@dataclass
+class VerificationResult:
+    filename: str
+    expected_hash: str
+    actual_hash: Optional[str]
+    exists: bool
+    matches: bool
+
+
+class VerifyDownload(Subcommand):
+    """Llama cli for verifying downloaded model files"""
+
+    def __init__(self, subparsers: argparse._SubParsersAction):
+        super().__init__()
+        self.parser = subparsers.add_parser(
+            "verify-download",
+            prog="llama verify-download",
+            description="Verify integrity of downloaded model files",
+            formatter_class=argparse.RawTextHelpFormatter,
+        )
+        setup_verify_download_parser(self.parser)
+
+
+def setup_verify_download_parser(parser: argparse.ArgumentParser) -> None:
+    parser.add_argument(
+        "--model-id",
+        required=True,
+        help="Model ID to verify",
+    )
+    parser.set_defaults(func=partial(run_verify_cmd, parser=parser))
+
+
+def calculate_md5(filepath: Path, chunk_size: int = 8192) -> str:
+    md5_hash = hashlib.md5()
+    with open(filepath, "rb") as f:
+        for chunk in iter(lambda: f.read(chunk_size), b""):
+            md5_hash.update(chunk)
+    return md5_hash.hexdigest()
+
+
+def load_checksums(checklist_path: Path) -> Dict[str, str]:
+    checksums = {}
+    with open(checklist_path, "r") as f:
+        for line in f:
+            if line.strip():
+                md5sum, filepath = line.strip().split("  ", 1)
+                # Remove leading './' if present
+                filepath = filepath.lstrip("./")
+                checksums[filepath] = md5sum
+    return checksums
+
+
+def verify_files(
+    model_dir: Path, checksums: Dict[str, str], console: Console
+) -> List[VerificationResult]:
+    results = []
+
+    with Progress(
+        SpinnerColumn(),
+        TextColumn("[progress.description]{task.description}"),
+        console=console,
+    ) as progress:
+        for filepath, expected_hash in checksums.items():
+            full_path = model_dir / filepath
+            task_id = progress.add_task(f"Verifying {filepath}...", total=None)
+
+            exists = full_path.exists()
+            actual_hash = None
+            matches = False
+
+            if exists:
+                actual_hash = calculate_md5(full_path)
+                matches = actual_hash == expected_hash
+
+            results.append(
+                VerificationResult(
+                    filename=filepath,
+                    expected_hash=expected_hash,
+                    actual_hash=actual_hash,
+                    exists=exists,
+                    matches=matches,
+                )
+            )
+
+            progress.remove_task(task_id)
+
+    return results
+
+
+def run_verify_cmd(args: argparse.Namespace, parser: argparse.ArgumentParser):
+    from llama_stack.distribution.utils.model_utils import model_local_dir
+
+    console = Console()
+    model_dir = Path(model_local_dir(args.model_id))
+    checklist_path = model_dir / "checklist.chk"
+
+    if not model_dir.exists():
+        parser.error(f"Model directory not found: {model_dir}")
+
+    if not checklist_path.exists():
+        parser.error(f"Checklist file not found: {checklist_path}")
+
+    checksums = load_checksums(checklist_path)
+    results = verify_files(model_dir, checksums, console)
+
+    # Print results
+    console.print("\nVerification Results:")
+
+    all_good = True
+    for result in results:
+        if not result.exists:
+            console.print(f"[red]❌ {result.filename}: File not found[/red]")
+            all_good = False
+        elif not result.matches:
+            console.print(
+                f"[red]❌ {result.filename}: Hash mismatch[/red]\n"
+                f"   Expected: {result.expected_hash}\n"
+                f"   Got:      {result.actual_hash}"
+            )
+            all_good = False
+        else:
+            console.print(f"[green]✓ {result.filename}: Verified[/green]")
+
+    if all_good:
+        console.print("\n[green]All files verified successfully![/green]")