Implement timeout for slow functions

[mdouze]

In Faiss, slow functions periodically call InterruptCallback::check()

https://github.com/facebookresearch/faiss/blob/main/faiss/impl/AuxIndexStructures.h#L152

The callback is arbitrary. It does nothing by default.
In python there is an InterruptCallback that checks if the user pressed Ctrl-C.

It would be useful to implement an InterruptCallback that breaks out of a function that lasts too long, eg. a clustering.

[mdouze]

1. C++ only first
2. support timeout from python

[mdouze]

For the C++ version it can be done with just one additional test.
It requires subclassing InterruptCallback to implement want_interrupt to return true if more than x seconds elapsed since the counter was reset.

Something like:

struct TimeoutCallback: InterruptCallback {
    double t0 = 0; 
    double timeout = 0; 
    void set_timeout(double t) {
        t0 = get_time(); 
        timeout = t; 
    } 
   
   bool want_interrupt () override {
      if (timeout == 0) {
         return false; 
      }
      if (get_time() - t0 > timeout) {
         timeout = 0;  
         return true; 
      } 
     return false;
   }

};

can be tested by running a very long k-means clustering (see Clustering object).