### Description
* follows the packaging approach according to the design document
* adds `ENABLE_TRAINING` boolean flag to `BUILD_DEFS`
* modifies `package.json` to include training submodule
* modifies build script to handle, validate, and minimize training WASM
artifacts
* adds the binding for the new backend with training enabled & the new
training artifacts
* adds training backend
* edits `index.ts` to use training backend depending on `BUILD_DEFS`
* edits `wasm-factory.ts` to use the training artifacts if necessary
### Motivation and Context
* we are in the process of adding web bindings to enable training.
* Adding the "glue" to allow onnxruntime-web to use the training WASM
artifacts is required for this work.
* Since BUILD_DEFS is defined and used at build time, I thought that it
made sense to bundle the changes to building in the same PR.
#### Related work
* #16521 allowed for training artifacts to be built
* #17333 must be merged in before this one
---------
Co-authored-by: Yulong Wang <7679871+fs-eire@users.noreply.github.com>
### Description
add target ort.webgpu.min.js
WebGPU is experimental feature, so I don't want to put webgpu into the
ort.min.js file. This change adds 2 ways for users to access ort-web
with webgpu:
- using script tag: by URL
`https://cdn.jsdelivr.net/npm/onnxruntime-web@1.15.0/dist/ort.webgpu.min.js`
( this URL is not ready yet )
- using `import()`: use `import { Tensor, InferenceSession } from
'onnxruntime-web/webgpu';` - 'onnxruntime-web/webgpu' instead of
'onnxruntime-web'
