Migrate tfjs-node backend to use bytes for strings

[dsmilkov]

With tensorflow/tfjs-core#1816, string tensors are stored encoded as bytes using Uint8Array[] in the backend.

We need to migrate the tfjs-node backend to align with this and be compatible with latest tfjs-core.

[TheNewSound]

• add possibility to cast 'string' tensor to 'int' (uint8?) tensor?

Example: I load up a string and want to convert lowercase to uppercase:

let t=tf.tensor([["ammmmmmmmmmmmmmmmmmmmmmmmz"], ["AZ"]],[2,1],'string');
t.print();
let b = tf.scalar(97);
let c = t.asType('int32').greaterEqual(b);
let d = c.asType('int32').mul(32);
t.sub(d).print();

[dsmilkov]

Will close this issue since it was fixed in tensorflow/tfjs-node#279

Regarding the question, can you try using plain JS String.toLowerCase? If that doesn't fit you need, feel free to file another issue.