Track all of our RNG offsets to avoid collisions

Created by: suchenzang

We have RNG seed offsets sprinkled through the codebase:

(base) √ metaseq % ag seed --py | grep +
cpu_tests/test_streaming_token_block_dataset.py:78:        shadow_rng = np.random.default_rng(2273 + seed)
cpu_tests/test_streaming_token_block_dataset.py:124:        shadow_rng = np.random.default_rng(2273 + seed)
metaseq/tasks/language_modeling.py:217:            with data_utils.numpy_seed(self.args.seed + epoch):
metaseq/tasks/streaming_language_modeling.py:316:            seed=1284 + self.args.seed,
metaseq/trainer.py:1052:        seed = self.cfg.common.seed + self.get_num_updates()
metaseq/data/streaming_token_block_dataset.py:96:            rng = np.random.default_rng(2273 + self.seed)
metaseq/data/iterators.py:524:                batches = shuffle_batches(list(batches), self.seed + epoch)
metaseq/data/iterators.py:532:                batches = shuffle_batches(batches, self.seed + epoch + self.shard_id)
metaseq/data/iterators.py:535:                batches = shuffle_batches(list(self.frozen_batches), self.seed + epoch)

Would be good to track these offset to avoid collisions, in cases we're assuming no collision/coupling via offsets.