Change tf loader API

[syt123450]

Previously, when loading a tensorflow frozen model in TensorSpace, it is required to provide two parameters: modelUrl and weightUrl. As TensorSpace uses tfjs's new loading API, weightUrl is no longer required, so new tensorflow load API will look like this temporary:

model.load( {
    type: "tensorflow",
    modelUrl: "/PATH_TO_MODEL/tensorflowjs_model.pb",
    outputsName: [ ... ],
} );

Next step is to change modelUrl to url, make tensorflow loader conform with keras loader and tfjs loader.

TODO:

• [x] Change loader
• [x] Change usage in examples
• [x] Change usage in playground
• [x] Modify docs