Use "float64" dtypes in DaskExecutor.fit_phase

[rjzamora]

Addresses NVTabular test failures with rapids-24.04 (pandas-2).

This PR does not "fix" the existing dtype-preservation problem:

            # TODO: constructing meta like this loses dtype information on the ddf
            # and sets it all to 'float64'. We should propagate dtype information along
            # with column names in the columngroup graph. This currently only
            # happens during intermediate 'fit' transforms, so as long as statoperators
            # don't require dtype information on the DDF this doesn't matter all that much

However, it makes pandas-2 behavior consistent with pandas<2.

[github-actions]

Documentation preview

https://nvidia-merlin.github.io/core/review/pr-378

[benfred]

lgtm!