Accept path to jinja2 template

[butsugiri]

• as-is: a user can only pass jinja2 template whose type is str
• to-be: a user can pass path to jinja2 template as well as str

This is convenient when we want to iterate multiple path_to_templates, like this:

{
    class_path: 'flexeval.JsonlChatDataset',
    init_args: {
    path: '/path/to/dataset.jsonl',
    input_template: path_to_template,
    },
},