breaking: rename distributions to distribution in TF/MXNet estimators

[laurenyu]

Issue #, if available:
#1473

Description of changes:
This was a typo that wasn't caught until after a couple releases, so the consensus was to wait until other breaking changes to fix this.

Testing done:
unit tests, linters

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[metrizable]

looks good!

[metrizable]

@ignorable style: it may be overkill to abstract functionality from _has_distribution_arg and modify_node, something like:

def _distribution_arg(self, node):
    for kw in node.keywords:
        if kw.arg == "distributions":
            return kw
    return None

def _has_distribution_arg(self, node):
    return self._distribution_arg(node) is not None

def modify_node(self, node):
    kw = self._distribution_arg(node)
    kw.arg == "distribution"

[laurenyu]

hmm...it kind of feels like overkill in this context, but I could see potential in the arg retrieval as a standalone utility function that's then used with multiple modifier classes