MPI nodes don't handle scalar args correctly

[kylosus]

Describe the bug
The dace program fails to compile if the integer args of MPI nodes (dst and tag) are anything other than symbols, symbol expressions and numbers.

To Reproduce

rank = dc.symbol('rank', dtype=dc.int64)

@dc.program
def func(A: dc.int32[N]):
    # dace.comm.Send(A[0], rank - 1, 0) # Works
    dace.comm.Send(A[0], abs(rank - 1), 0)

The program fails to compile with the following error:

ValueError: Node type "Send" not supported for promotion

Same behavior in other scenarios:

# ...
a = 0
a = rank
a = A[0]
dace.comm.Send(A[0], a, 0)

Desktop (please complete the following information):

• Latest DaCe master branch

Possible fix:
The code below should check if the sdfg.arrays entry for the corresponding arg is a Scalar when given an str and fall to the last branch.

dace/dace/frontend/common/distr.py

Lines 424 to 436 in f4b4d01

	dst_range = None
	if isinstance(dst, tuple):
	dst_name, dst_range = dst
	dst_node = state.add_read(dst_name)
	elif isinstance(dst, str) and dst in sdfg.arrays.keys():
	dst_name = dst
	dst_node = state.add_read(dst_name)
	else:
	storage = desc.storage
	dst_name = _define_local_scalar(pv, sdfg, state, dace.int32, storage)
	dst_node = state.add_access(dst_name)
	dst_tasklet = state.add_tasklet('_set_dst_', {}, {'__out'}, '__out = {}'.format(dst))
	state.add_edge(dst_tasklet, '__out', dst_node, None, Memlet.simple(dst_name, '0'))