code: function getPartials(file, sourcePath, templates, customPartialName, callback) {
    const { partials } = templates;
    if (file.meta.customLayout === undefined) {
      async.nextTick(callback, null, partials);
      return;
    }
  
    const customLayoutPath = path.join(
      sourcePath,
      path.resolve(path.sep, file.meta.filePath.dir, file.meta.customLayout)
    );
    read(customLayoutPath, ReadMode.UTF8, (error, data) => {
      if (error) {
        console.error(`${file.path}: Could not load the custom layout`);
        async.nextTick(callback, error);
        return;
      }
  
      const customPartials = { [customPartialName]: data };
      async.nextTick(callback, null, Object.assign({}, partials, customPartials));
    });
  }
docs: Retrieve the template partials.