Optimize `to_cupy` and `values`

[vyasr]

Currently series.values and especially series.to_cupy() are substantially slower than cupy.asarray(series).

In [2]: s = cudf.Series(range(10000))

In [3]: %timeit s.values
81.4 µs ± 1.68 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

In [4]: %timeit cp.asarray(s)
19.1 µs ± 168 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

In [5]: %timeit s.to_cupy()
349 µs ± 75.2 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)

There are at least two obvious potential culprits in Frame._to_array (the underlying method for to_cupy):

• It always performs an extra allocation, even when copy=False.
• It performs dtype inference using find_common_dtype, which is slow (and slower for DataFrames with many columns):

In [11]: df = cudf.DataFrame({'a': [1], 'b': [3.], 'c': ['a']})

In [12]: %timeit cudf.utils.dtypes.find_common_type([col.dtype for col in df._data.values()])
53.6 µs ± 530 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

In [13]: df = cudf.DataFrame({'a': [1], 'b': [3.]})

In [14]: %timeit cudf.utils.dtypes.find_common_type([col.dtype for col in df._data.values()])
39.8 µs ± 1.01 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

The implementation of values drops down to ColumnBase.values and requires some deeper consideration. However, since we use .values frequently internally (and we occasionally use to_cupy) we are likely giving up a lot of performance. We should profile these functions to determine the bottlenecks, and if there are valid reasons for them we should establish some policies on how to select the right function to use when performing these conversions to arrays internally. While this exact analogy does not hold for DataFrame (because that doesn't support the conversion to an array), any optimization that we make for Series will likely also help speed up DataFrame operations.

[github-actions]

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[vyasr]

CC @mroeschke (might be of interest given some of your recent comments about converting to device and all the internal reworkings you're doing with our columns)