Add RECIPROCAL pointwise op support

[rsuderman]

No description provided.

[sjain-stanford]

RECIPROCAL is registered as arity-1 and has a lit test, but is NOT added to the pointwise_unary_ops.cpp GENERATE list or its switch statement. Every other non-boolean unary op PR (NEG, FLOOR, EXP, LOG, ERF, ELU_FWD) correctly adds to this sample test. This appears to be an oversight.

Fix: Add PointwiseAttr::Mode::RECIPROCAL to the GENERATE list and add:

case PointwiseAttr::Mode::RECIPROCAL: {
  double xD = static_cast<double>(x);
  y = 1.0 / xD;
  break;
}