Support multiple node generation for single trial

[mgarrard]

Summary: This diff enables multiple nodes to be used to generate a single batch trial. Right now the limitations are that: (1) currently each node only contributes 1 gr to the trial -- we will extend this to n grs in a future diff (2) we don't support multiple transition criterion creating individual forks to another trial -- all trial blocking tc must be met in order to move to the next step --> we don't have a usecase for this right now, so i think it's okay to wait on this. The current implementation should be quite easy to extend to support this.

To do this we added: (1) trial transitions property (2) generation node dict property (3) should_move_trials method and (4) a temp gen method to derisk the dev

Differential Revision: D56743651