Resetting variables whenever a new prompt is sampled.

[adieyal]

This relates to this issue: adieyal/sd-dynamic-prompts#517

Here is the relevant code:
https://github.com/adieyal/dynamicprompts/blob/main/src/dynamicprompts/samplers/base.py#L71-L75

    def _get_sequence(
        self,
        command: SequenceCommand,
        context: SamplingContext,
    ) -> StringGen:
        tokens, context = context.process_variable_assignments(command.tokens)
        sub_generators = [context.generator_from_command(c) for c in tokens]


        while True:
            yield rotate_and_join(sub_generators, separator=command.separator)

variable_assignment takes place outside of the while loop. Without creating the sub_generators, I'm not sure how to make this work. Any suggestions? @akx