This came up while debugging memory corruption in the ngraph-tf tests: plaidml_ops_replicate was holding onto
a reference to a temporary that was destroyed while the reference was still live.  Replacing that code with a
call to get_argument() eliminated the temporary and fixed the issue.  And in general, there's no reason to be
allocating temporary vectors when we're pulling out a single argument, so this change replaces the other
sequences of get_arguments().at(), too.