[Data] add reset pandas index when merge sorted blocks

[tespent]

Why are these changes needed?

The index of pandas blocks are reset after all block processing, but not for sort currently. This resulted in a lot of other code being broken. For example in get_bounds of random access dataset, it uses the index to get the first and the last row (L286), but without resetting index after sort, it gets random rows.

ray/python/ray/data/random_access_dataset.py

Lines 283 to 289 in 9472ae2

	def _get_bounds(block, key):
	if len(block) == 0:
	return None
	b = (block[key][0], block[key][len(block) - 1])
	if isinstance(block, pa.Table):
	b = (b[0].as_py(), b[1].as_py())
	return b

Related issue number

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• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[MissiontoMars]

@raulchen