Prediction channel selection

[yrkim98]

In prediction, there's a couple additions we need to make in order to ensure the correct channel is predicted/displayed in the plugin

1. When the user selects a single channel image, the channel selection dropdown should be disabled, and the channel override should default to channel 0.
2. When the user selects a multi channel image, the channel selection dropdown should be enabled, and we should make sure the user has selected a channel before continuing.

[thao-do]

@yrkim98 for case 1: is there a strong need technically to make the channel selection dropdown being disabled which I interpret it as the whole dropdown container would look disabled, the default display (channel 0) therefore also look disabled, and user wouldn't be able to interact with it at all like looking at the dropdown list with the one item it has? It might communicate the idea that there is no option at all available to user, while it has, albeit just one. I agree with the overwrite and default to channel 0, but suggest leaving the dropdown still enabled.

for case 2: I noted this in my smoke-test report if a user didn't select a channel for run prediction, beside the error pops up (also reported in the form) there is also unexpected app behavior:

without channel input prediction still happens but then returned error and clear out the current image on-screen while the rest of the UI unchanged >> unexpected