{relay,topi}.reinterpret operator

[ajtulloch]

Motivation

It's useful to expose the tvm::reinterpret functionality to Relay/TOPI users, as
this allows them to build (fused) operators leveraging the bitwise
reinterpretation of tensor elements. An example is approximate transcendental
functions, which can be implemented similar to:

    def C(x):
        return relay.expr.const(x, "float32")

    def approx_exp(x):
        x = relay.minimum(relay.maximum(x, C(-88.0)), C(88.0))
        x = C(127.0) + x * C(1.44269504)
        xf = relay.floor(x)
        i = relay.cast(xf, "int32")
        x = x - xf
        Y = C(0.99992522) + x * (C(0.69583354) + x * (C(0.22606716) + x * C(0.078024523)))
        exponent = relay.left_shift(i, relay.expr.const(23, "int32"))
        exponent = relay.reinterpret(exponent, "float32")
        return exponent * Y

    def approx_sigmoid(x):
        # <2.0e-5 absolute error over [-5, 5]
        y = approx_exp(x)
        return y / (y + C(1.0))

    def approx_tanh(x):
        # <4.0e-5 absolute error over [-5, 5]
        x = x * C(2.0)
        y = approx_exp(x)
        return (y - C(1.0)) / (y + C(1.0))

See unit tests for implementations of these approximate transcendentals.

[ajtulloch]

cc @tqchen re: changes to CodeGenC (supporting reinterpret intrinsic).

[ajtulloch]

cc @yidawang, may be of interest.

[tqchen]

Thanks @ajtulloch !