Support nested tuple unpack in CuPy JIT

[asi1024]

This PR supports nested tuple unpack in CuPy JIT.

Example:

@cupyx.jit.rawkernel()
def f(x, y, z):
    i = cupyx.jit.threadIdx.x
    (x[i], y[i]), z[i] = (z[i], x[i]), y[i]

x = cupy.full((5), 10, dtype=cupy.int32)
y = cupy.full((5), 20, dtype=cupy.int32)
z = cupy.full((5), 30, dtype=cupy.int32)
f((1,), (5,), (x, y, z))

Generated kernel:

extern "C" __global__ void f(CArray<int, 1, true, true> x, CArray<int, 1, true, true> y, CArray<int, 1, true, true> z) {
  unsigned int i;
  i = threadIdx.x;
  {
    thrust::tuple<thrust::tuple<int, int>, int> _temp0 = thrust::make_tuple(thrust::make_tuple(z[i], x[i]), y[i]);
    {
      thrust::tuple<int, int> _temp1 = thrust::get<0>(_temp0);
      x[i] = thrust::get<0>(_temp1);
      y[i] = thrust::get<1>(_temp1);
    }
    z[i] = thrust::get<1>(_temp0);
  }
}

Related discussion: #5293 (comment) (cc/ @eternalphane)

[asi1024]

Previously thrust::tie has been used to unpack tuple, but it cannot be used for nested tuples. So we decided to use thrust::get<i> to rvalues instead.

Example:

@cupyx.jit.rawkernel()
def f(x, y):
    i = cupyx.jit.threadIdx.x
    x[i], y[i] = y[i], x[i]

Previously, the following kernel has been generated:

extern "C" __global__ void f(CArray<int, 1, true, true> x, CArray<int, 1, true, true> y) {
  unsigned int i;
  i = threadIdx.x;
  thrust::tie(x[i], y[i]) = thrust::make_tuple(y[i], x[i]);
}

From this PR, the following kernel will be generated:

extern "C" __global__ void f(CArray<int, 1, true, true> x, CArray<int, 1, true, true> y) {
  unsigned int i;
  i = threadIdx.x;
  {
    thrust::tuple<int, int> _temp0 = thrust::make_tuple(y[i], x[i]);
    x[i] = thrust::get<0>(_temp0);
    y[i] = thrust::get<1>(_temp0);
  }
}

[emcastillo]

Jenkins, test this please

[asi1024]

Sorry, I have forgotten remove debug logging.
Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit 69414b5, target branch master) failed with status FAILURE.

[emcastillo]

Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit da50f65, target branch master) succeeded!