Skip conversion to a tmp model for trtllm if it's present. Add a way to pass parameters to the first stage

[Kipok]

trtllm conversion consists of two steps and most of the time only the second needs to be rerun if something changes (e.g. max_length or max batch size). We should by default reuse an existing tmp checkpoint if it's present but keep an option to override

There is also currently no way to pass parameters to the first stage of conversion, which we should fix