Code cleanup: Replaced 'idx' with '_'. Removed unused parameter & unused import

[faeezshaikh]

1. The idx variable is not used in the code after it is defined, so it can be removed without affecting the behavior of the code. The idx should be replaced with _. This will indicate that the loop variable is not used, while still iterating over num_examples times.

2. Removed unused parameter from the get_readers(). The num_readers_per_worker parameter is not being used inside the get_readers function, and it appears to be an unused parameter. The function creates one reader per worker, so num_readers_per_worker does not seem to have any effect on the behavior of the function. It's possible that this parameter was intended to be used in some way but was later removed or overlooked. Therefore, num_readers_per_worker appears to be an unnecessary parameter and can be removed from the function signature.

3. Removed unused import LocalFileSystem. LocalFileSystem from fsspec.implementations.local is imported but not used in the code.

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