[Data] Fix bug where _StatsActor errors with PandasBlock

[bveeramani]

Why are these changes needed?

tl;dr: StatsActor errors if your dataset contains pandas blocks.

Results from write tasks are stored in pandas blocks.

ray/python/ray/data/_internal/planner/plan_write_op.py

Lines 22 to 30 in 7c44833

	def fn(blocks: Iterator[Block], ctx) -> Iterator[Block]:
	# NOTE: `WriteResult` isn't a valid block type, so we need to wrap it up.
	import pandas as pd
	block = pd.DataFrame(
	{"write_result": [datasource.write(blocks, ctx, **write_args)]}
	)
	return [block]

The problem is that Ray expects stats to be an int or float:

ray/python/ray/data/_internal/stats.py

Line 225 in 7c44833

self.bytes_allocated.set(stats["obj_store_mem_alloc"], tags)

But PandasBlock.size_bytes() returns a NumPy scalar.

ray/python/ray/data/_internal/pandas_block.py

Line 241 in 7c44833

return self._table.memory_usage(index=True, deep=True).sum()

This PR fixes the issue by converting the NumPy scalar to an int.

Related issue number

Fixes #40480

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[Zandew]

Thanks for the fix!