Collect instruction counts using Callgrind.
  Unlike wall times, instruction counts are deterministic (modulo
  non-determinism in the program itself and small amounts of
  jitter from the Python interpreter.) This makes them ideal for
  detailed performance analysis. This method runs *stmt* in a
  separate process so that Valgrind can instrument the program.
  Performance is severely degraded due to the instrumentation,
  however this is ameliorated by the fact that a small number of
  iterations is generally sufficient to obtain good measurements.

  In order to to use this method *valgrind*, *callgrind_control*,
  and *callgrind_annotate* must be installed.

  Because there is a process boundary between the caller (this
  process) and the *stmt* execution, *globals* cannot contain
  arbitrary in-memory data structures. (Unlike timing methods)
  Instead, globals are restricted to builtins, *nn.Modules*'s, and
