Add bincount method to array class

[anupamme]

Fixes #1786

Add bincount method to MLX to count occurrences of each value in an array.

• mlx/array.cpp: Implement bincount method using std::unordered_map to store counts and return an array with the counts of each value in the input array.
• mlx/array.h: Declare bincount method in the array class.
• python/src/array.cpp: Add bincount method to the Python API for the array class and implement it using the bincount method from mlx/array.cpp.
• python/tests/test_array.py: Add test cases for the bincount method in the array class to verify its correctness with various input arrays.

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[angeloskath]

Hi, the linked issue has a much more concise and correct implementation as follows

def bincount(input, max_val: int):
    return mx.zeros(max_val, dtype=mx.int32).at[input].add(1)

this also runs in the GPU and doesn't require evaluating input before computing the bin count.