iterate_batches in mlx_lm's Lora trainer is discarding the remainder dataset items (modulo batch size)

The current implementation of iterate_batches produces batches for all but the remaining N items in the dataset (where N is less than the batch size). So, if your dataset size is a multiple of the batch size, you will eventually train on every item in the dataset. However, if it is not, the remainder will never be included in what is trained, regardless of how many iterations you set.

Below is very minimal test case that replicates this (it generates batches of a total size of at most 46 items without finding the remaining data before stopping):

from unittest.mock import MagicMock
from mlx_lm.tuner.trainer import iterate_batches
import mlx.core as mx

class TestBatching(unittest.TestCase):
    def test_batch_remainder(self):
        tokenizer = MagicMock()
        tokenizer.eos_token_id = 9
        tokenizer.encode = lambda x: [1,] if x == "foo" else [7,]
        
        #The dataset is 20 "foo"s and 3 "bar"s (the remainder) 
        dataset = ["foo"] * 20 + ["bar", "bar", "bar"]
        batch_size = 10
        found_remainders = False
        for idx, (batch_in, _, _) in enumerate(iterate_batches(dataset, tokenizer, batch_size, max_seq_length=2048,
                                                               train=True)):
            found_remainders = mx.any(batch_in == mx.full(batch_in.shape, 7))
            if found_remainders or ((idx + 1) * batch_size) > len(dataset) * 2:
                break
        self.assertTrue(found_remainders.item())

Start with iterate_batches in mlx_lm/tuner/trainer.py and reproduce the issue using the provided TestBatching case with a 23-item dataset and batch size 10. Done means the generated batches eventually include the three remainder items, and the regression test passes.