Set_item: improve error message on un-broadcastable lhs & rhs

[manopapad]

The following:

x = cn.arange(4).reshape((2,2)).astype(np.float64)
inds = (
    cn.array([0, 1]),
    cn.array([0, 1]),
)
x[inds] = cn.zeros((3,))

produces an assertion failure:

Traceback (most recent call last):
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legion_top.py", line 440, in legion_python_main
    run_path(args[start], run_name='__main__')
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legion_top.py", line 255, in run_path
    exec(code, module.__dict__, module.__dict__)
  File "a.py", line 43, in <module>
    x[inds] = cn.zeros((3,))
  File "/Users/mpapadakis/cunumeric/cunumeric/coverage.py", line 107, in wrapper
    return func(*args, **kwargs)
  File "/Users/mpapadakis/cunumeric/cunumeric/array.py", line 105, in wrapper
    return func(*args, **kwargs)
  File "/Users/mpapadakis/cunumeric/cunumeric/array.py", line 1513, in __setitem__
    self._thunk.set_item(key, value._thunk)
  File "/Users/mpapadakis/cunumeric/cunumeric/deferred.py", line 81, in wrapper
    return func(*args, **kwargs)
  File "/Users/mpapadakis/cunumeric/cunumeric/deferred.py", line 734, in set_item
    rhs_tmp = rhs._broadcast(index_array.base.shape)
  File "/Users/mpapadakis/cunumeric/cunumeric/deferred.py", line 643, in _broadcast
    assert result.shape[dim] == 1
AssertionError

while this variant:

x = cn.arange(4).reshape((2,2)).astype(np.float64)
inds = (
    cn.array([0, 1]),
    cn.array([0, 1]),
)
x[inds] = cn.zeros((1,3))

makes it to the task launch stage before failing:

Traceback (most recent call last):
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legion_top.py", line 440, in legion_python_main
    run_path(args[start], run_name='__main__')
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legion_top.py", line 255, in run_path
    exec(code, module.__dict__, module.__dict__)
  File "a.py", line 43, in <module>
    x[inds] = cn.zeros((1,3))
  File "/Users/mpapadakis/cunumeric/cunumeric/coverage.py", line 107, in wrapper
    return func(*args, **kwargs)
  File "/Users/mpapadakis/cunumeric/cunumeric/array.py", line 105, in wrapper
    return func(*args, **kwargs)
  File "/Users/mpapadakis/cunumeric/cunumeric/array.py", line 1513, in __setitem__
    self._thunk.set_item(key, value._thunk)
  File "/Users/mpapadakis/cunumeric/cunumeric/deferred.py", line 81, in wrapper
    return func(*args, **kwargs)
  File "/Users/mpapadakis/cunumeric/cunumeric/deferred.py", line 763, in set_item
    copy.execute()
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legate/core/operation.py", line 167, in execute
    self._context.runtime.submit(self)
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legate/core/runtime.py", line 1048, in submit
    self.flush_scheduling_window()
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legate/core/runtime.py", line 1041, in flush_scheduling_window
    self._schedule(ops)
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legate/core/runtime.py", line 1031, in _schedule
    strategies.append(partitioner.partition_stores())
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legate/core/solver.py", line 404, in partition_stores
    for c in op.constraints:
  File "/Users/mpapadakis/legate.core/install/lib/python3.8/site-packages/legate/core/operation.py", line 790, in constraints
    raise ValueError(
ValueError: Each input must have the same shape as the corresponding target indirect field, but got (2, 3) and (2,)