Presuming that all other means of controlling an aircraft are unavailable to a pilot, can a pilot land a commercial airliner safely by using only the throttles and are pilots trained to do so?
Not safely, no. According to the Wikipedia article on UA Flight 232, with only throttle control, it is impossible to "control airspeed independent from sink rate", because there's no control of angle of attack.
In 232's case, that meant:
On final descent, the aircraft was going 220 knots and sinking at 1,850 feet per minute, while a safe landing would require 140 knots and 300 feet per minute
I.e., even going 50% above safe landing speed the descent rate was 6 times too high. This is nowhere near safe, and it was something of a miracle that 185 out of the 296 souls aboard survived.
After that accident, the NTSB decided training for such a case wasn't practical.
One successful landing using differential throttle alone has been made by an airliner (actually an A300 cargo variant). With hydraulics out due to a SAM missile hit, they brought the plane down to a safe landing with no further damage, using differential throttle alone.