Rework on SerializableModuleGenerator for abstract type.

[sequencer]

The reason this API need to be rewritten is because SerializableModuleGenerator[_, _] cannot live in a collection(e.g. Seq, Map), since the types of SerializableModule and SerializableModuleParameter are unbounded.

Basically I replaced the type bound between SerializableModuleParameter and SerializableModule with concrete inner type: SerializableModule#SerializableModuleParameter. And type bound is not required anymore.

But I'm still struggling with the serialization version of SerializableModuleGenerator[_]:

FIXME: could not find implicit value for parameter e: upickle.default.Reader[Seq[chisel3.experimental.SerializableModuleGenerator[_ <: chisel3.experimental.SerializableModule]]]

This is a type gymnastics that I have been struggled for my evening...

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[sequencer]

I encountered the deserialization problem in this PR: module and parameter are read from json via ujson at runtime.

which means we cannot use whichbox(macro) to construct it at compile time. So the only solution is runtime reflection.

I spent a whole day to find if there are any solution to instantiate a abstract class with type and implicit val in it. But found nothing...

I comes to doubt that there is no possibility for constructing the SerializableModuleGenerator at Scala runtime.

:melt:

[sequencer]

Maybe use scala plugin to traverse code and find all SerializableModule and create a companion concrete class SerializableGenerator, so that the scala reflection will have concrete class to construct.

[sequencer]

Ok this is still impossible even with the scala compiler plugin.
Since for Seq[SerModule], elements of which are from runtime, and there seems to be no way to pass the implicit parameter during the runtime...

[sequencer]

I wish Chisel could support Scala3 in the future, so that we can have the dependent type to resolve such issues.... We may need to consider switching to runtime json serdes lib like json4s

[sequencer]

@jackkoenig This was the draft PR which I mentioned in the dev meeting.