It’s not possible with the torchaudio library as of now.  You could preprocess your files with a shell script that uses sox to do this.
#!/bin/bash

TMPDIR=/tmp/sox

for fn in $(find . -name "*.wav"); do
  TMPFILE=$TMPDIR/$(basename $fn)
  sox $fn $TMPFILE rate 16000
  mv $TMPFILE $fn
done