Adding custom dataset file

[goswamig]

What does this PR do?

Fixes # (658)
The custom dataset file was removed probably by mistake during this PR #650

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[goswamig]

Without this

 >>> from llama_recipes.utils.dataset_utils import get_dataloader
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ubuntu/llama-recipes/src/llama_recipes/utils/__init__.py", line 5, in <module>
    from llama_recipes.utils.dataset_utils import *
  File "/home/ubuntu/llama-recipes/src/llama_recipes/utils/dataset_utils.py", line 7, in <module>
    from llama_recipes.datasets import DATASET_PREPROC, get_custom_dataset
  File "/home/ubuntu/llama-recipes/src/llama_recipes/datasets/__init__.py", line 8, in <module>
    from llama_recipes.datasets.custom_dataset import get_custom_dataset
ModuleNotFoundError: No module named 'llama_recipes.datasets.custom_dataset'

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[goswamig]

@mreso Can you please take a look?

[mreso]

Yep, the classic "new file missing from PR...."
Thanks for the fix! LGTM