Add init_position argument to UniformGenerator

[saitcakmak]

Summary: SobolGenerator uses init_position to ensure that when the model is reconstructed, it resumes candidate generation from where it was left (rather than starting from the beginning of the sequence). Without this, when deduplicate=False, the model would generate the same points it has already generated, which would lead to different candidate generation behaviors depending on how often the model was reconstructed. This is undesirable as we want the model to resume generation rather than repeating from scratch.

Prior to this diff, UniformGenerator did not have init_position, so had this exact issue. We fix it here.

Differential Revision: D61622058