No appeal in certain cases when accused pleads guilty.

Notwithstanding anything contained in section 374, where an accused person has pleaded guilty and has been convicted on such plea, there shall be no appeal,---

1. (a) if the conviction is by a High Court; or
2. (b) if the conviction is by a Court of Session, Metropolitan Magistrate or Magistrate of the first or second class, except as to the extent or legality of the sentence.