copy index values to host for iteration

[bryevdv]

cc @philippjfr @jlstevens

This PR adds some missing .values_host calls necessary on cudf indices before iteration. It fixes the following broken tests:

FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_1D_reduce_hm - AttributeError: 'StringIndex' object has no attribute 'value_host'
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_1D_reduce_hm_alias - AttributeError: 'StringIndex' object has no attribute 'value_host'
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_1D_reduce_ht - AttributeError: 'StringIndex' object has no attribute 'value_host'
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_2D_reduce_hm - AttributeError: 'StringIndex' object has no attribute 'value_host'
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_2D_reduce_ht - AttributeError: 'StringIndex' object has no attribute 'value_host'

[bryevdv]

This still leaves the following broken cudf tests, most (all?) of which seem to relate to current lack of string support in cudf:

FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_expanded_dimvals_ht - TypeError: unsupported operand type(s) for -: 'str' and 'str'
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_groupby - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_groupby_alias - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_groupby_dynamic - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_groupby_dynamic_alias - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_index_item_table - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_index_row_age - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_index_row_gender_female - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_index_rows_gender_male - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_index_value1 - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_index_value2 - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_sample_hm - holoviews.core.data.interface.DataError: Following columns specified as dimensions...
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_sample_hm_alias - holoviews.core.data.interface.DataError: Following columns specified as dime...
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_sample_ht - holoviews.core.data.interface.DataError: Following columns specified as dimensions...
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_select_rows_gender_male - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_select_rows_gender_male_alias - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_value_dim_index - NotImplementedError: String Arrays is not yet implemented in cudf
FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_value_dim_scalar_index - NotImplementedError: String Arrays is not yet implemented in cudf

I'm not sure how you might like to handle this since it seems most or all of these tests are inherited from a test base class, so I am not sure the simplest way to disable them.

[bryevdv]

There are a few failing tests above with:

FAILED holoviews/tests/core/data/testcudfinterface.py::cuDFInterfaceTests::test_dataset_sample_hm - holoviews.core.data.interface.DataError: Following columns specified as dimensions...

And I am not sure those are string-related. Might be actual failures, but someone more knowledgeable than me will have to comment.

[bryevdv]

@philippjfr @jlstevens the test failures appear unrelated?

[philippjfr]

Yeah, seeing those on another PR as well. Will check out this PR this evening on my GPU machine.

[philippjfr]

Merging and will do a round of testing on my GPU machine in a minute.