GH-48625: [Python] Add temporal unit checking in NumPyDtypeUnifier

[HyukjinKwon]

Rationale for this change

This is to address a todo:

arrow/python/pyarrow/src/arrow/python/inference.cc

Line 258 in de6eb89

// TODO: check that units are all the same

When users mix numpy.datetime64 values with different units (e.g., datetime64[s] and datetime64[ms]) in a single array, PyArrow previously produced a confusing error message

What changes are included in this PR?

• Added datetime64 unit validation in NumPyDtypeUnifier::Observe_DATETIME()
• Added InvalidDatetimeUnitMix() method
• Updated NumPyDtypeUnifier::Observe() to check units for same-type comparisons
• Updated existing test test_array_from_different_numpy_datetime_units_raises
• Removed the TODO comment (now implemented)

Are these changes tested?

Manually tested, and unittests were added.

Are there any user-facing changes?

Yes. It produces a better error message. For example,

import pyarrow as pa
import numpy as np
pa.array([np.datetime64('2020-01-01', 's'), np.datetime64('2020-01-02', 'ms')])

Before:

pyarrow.lib.ArrowNotImplementedError: Expected np.datetime64 but got: timestamp[ms]

After:

pyarrow.lib.ArrowInvalid: Cannot mix NumPy datetime64 units s and ms

• GitHub Issue: [Python] Add temporal unit checking in NumPyDtypeUnifier #48625

[github-actions]

⚠️ GitHub issue #48625 has been automatically assigned in GitHub to PR creator.

[AlenkaF]

LGTM, thanks!
@rok, short look, in case you have time.

[rok]

Looks good, just a minor nit.

[rok]

Numpy has other time units. Could we perhaps print which was used in case this is hit instead of unknown? I don't know if this is practical, just asking.

[HyukjinKwon]

Let me check and update it!

[HyukjinKwon]

I think it makes sense. Here is the example that makes sense:

import pyarrow as pa
import numpy as np
# `ps` unsupported but `s` supported.
pa.array([np.datetime64('2020', 'ps'), np.datetime64('2020', 's')])

Before:

pyarrow.lib.ArrowNotImplementedError: Unsupported datetime64 time unit

After:

pyarrow.lib.ArrowInvalid: Cannot mix NumPy datetime64 units ps and s