Prepare support for more checkpoint file formats

src/checkpoint.zig

@@ -30,21 +30,23 @@ weights: struct {
 },
 
 pub fn init(allocator: std.mem.Allocator, model_path: []const u8) !Self {
-    return try readLegacy(allocator, model_path);
-}
-
-pub fn readLegacy(allocator: std.mem.Allocator, model_path: []const u8) !Self {
-    const path = try std.fs.path.join(
+    const legacy_path = try std.fs.path.join(
         allocator,
         &[_][]const u8{ model_path, "checkpoint_legacy.bin" },
     );
 
-    defer allocator.free(path);
+    defer allocator.free(legacy_path);
+
+    const legacy_file = std.fs.cwd().openFile(legacy_path, .{}) catch null;
 
-    const file = try std.fs.cwd().openFile(path, .{});
+    defer if (legacy_file) |file| file.close();
 
-    defer file.close();
+    if (legacy_file) |file| return try readLegacy(allocator, file);
+
+    return error.CheckpointFileNotFound;
+}
 
+pub fn readLegacy(allocator: std.mem.Allocator, file: std.fs.File) !Self {
     const embedding_size: usize = @intCast(try file.reader().readIntLittle(i32));
     const ffn_hidden_size: usize = @intCast(try file.reader().readIntLittle(i32));
     const n_layers: usize = @intCast(try file.reader().readIntLittle(i32));