Merge Search & State into new "Generator" type

[RyanUnderhill]

  // Original version
  auto search = params.CreateSearch();
  auto state = model->CreateState(search->GetSequenceLengths(), params);

  while (!search->IsDone()) {
    search->SetLogits(state->Run(search->GetSequenceLength(), search->GetNextTokens()));
    search->SelectTop();
  }

  // Generator version
  auto generator = model->CreateGenerator();

  while (!generator->IsDone()) {
    generator->ComputeLogits();
    generator->AppendNextToken_Top();
  }

[yufenglee]

could you please also change SearchParams to GeneratorParams?

[yufenglee]

Append doesn't look like a good name to me. generator actually generate next token. How about GenerateNextToken

[yufenglee]

And can we make TopP a parameter instead of part of the name?

[RyanUnderhill]

I named it that since you first compute the logits, then append the next token based on the logits.
I initially had AddNextToken but 'Add' was less correct than 'Append' since it's appending tokens, not adding them.
Did you have a better name?

[yufenglee]

It is a process of generate next token. Add or Append doesn't reflect the action, i think.

[yufenglee]

could you please update this also