TFJS Debugger Feature Request - Disable unavailable backends

[jenperson]

Issue:

Currently, the backend dropdown menu is the same for all models regardless of available backends. There is no error shown. As a result, you can click "Run" before any error is surfaced.

For example, the Text Toxicity model does not support WASM. Once the debugger is running, a pop-up appears with the following error:

Failed to run model: Kernel 'SparseToDense' not registered for backend 'wasm'

Feature request:

Only display available backends in the dropdown menu.