I am not sure if there can be a definitive answer given to this question.  Much of it is going to depend on how an individual perceives Forster's writing, the mystery of the caves, and the depiction itself.  On one hand, Forster leaves quite ambiguous what happened in the caves.  There is a certain amount of vagueness present about what happened in the caves.  Forster alludes to the idea that this mystery is a part of "India."  This construction of the Indian mystique could be a very good example of a stereotype because of its capitulation to the idea of the "mysterious Orient" and other cultural interpretations.  Yet, Forster could simply be trying to bring out a sense of the unknown and it happened to be the Indian Marabar Caves as a setting for this.  Another level where there could be a debate about stereotypes might be in the characters, themselves.  Adela is shown to be a typically sexually repressed British woman, while Aziz is depicted as a sensuous Indian man who has little difficulty articulating such desires.  In this light, this could be a stereotype in its own right.  The sexually repressed Westerner and the libidinal driven Easterner.  The collision of such stereotypes results in the call of rape.  This might be interesting to note that the crash between both realities might result in a "cultural rape" where voice is taken from both sides, favoring the chatter of stereotypes.  As said, I am not sure that there can be a definitive answer here, but more thought being presented.