replace auto with auto& ref for cast<&>

[karthikeyann]

While debugging a 'scalar out of bounds memory issue' in compiled binary ops, found that auto is used instead of auto& for dynamic/static_cast<&>.
Using auto, causes to create a copy. (calls copy constructor).
But the intention of cast<&> is to create a ref variable.

This PR addresses several use cases in libcudf code of auto for dynamic/static_cast<&>.

[karthikeyann]

@davidwendt There is a comment mentioning this issue.

cudf/cpp/include/cudf/detail/indexalator.cuh

Line 430 in 8aaf8e6

// note: using static_cast<scalar_type_t<IndexType> const&>(index) creates a copy

[karthikeyann]

rerun tests

[davidwendt]

Nice work.

[karthikeyann]

Ran all libcudf tests with --rmm_mode=cuda

The following tests FAILED:
2 - SCALAR_TEST (Failed)
22 - BINARY_TEST (Failed)
32 - COPYING_TEST (Failed)
39 - MERGE_TEST (Failed)
46 - FACTORIES_TEST (Failed)

With this PR, still
The following tests FAILED:
2 - SCALAR_TEST (Failed)
32 - COPYING_TEST (Failed)
39 - MERGE_TEST (Failed)
46 - FACTORIES_TEST (Failed)

Created a new issue #8883 to track this.

[harrism]

rerun tests

[codecov]

Codecov Report

Merging #8866 (c07c179) into branch-21.10 (18f7c01) will decrease coverage by 0.08%.
The diff coverage is n/a.

❗ Current head c07c179 differs from pull request most recent head b4deb5d. Consider uploading reports for the commit b4deb5d to get more accurate results

@@               Coverage Diff                @@
##           branch-21.10    #8866      +/-   ##
================================================
- Coverage         10.67%   10.59%   -0.09%     
================================================
  Files               110      116       +6     
  Lines             18271    19037     +766     
================================================
+ Hits               1951     2017      +66     
- Misses            16320    17020     +700     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/categorical.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/column.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/lists.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/numerical.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/string.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/struct.py	0.00% <ø> (ø)
python/cudf/cudf/core/dataframe.py	0.00% <ø> (ø)
python/cudf/cudf/core/frame.py	0.00% <ø> (ø)
... and 75 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d7ba345...b4deb5d. Read the comment docs.

[harrism]

@gpucibot merge