Consider adding a warning when a cudaq.State is used as a buffer, causing copying of the state to host

[annagrin]

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Describe the bug

Some python statements cause cudaq.State act to as a buffer, causing copying of the data from the device to the host. Should we add a warning in those cases?

Steps to reproduce the bug

import numpy as np
import cudaq

c = np.array([1, 2j, 3, 4j, 5, 6j, 7, 8j], dtype=np.complex64)
state = cudaq.State.from_data(c)
state = state/np.linalg.norm(state) # Copies the state to host twice

@cudaq.kernel
def kernel(vec: list[complex]):
    q = cudaq.qvector(vec)

counts = cudaq.sample(kernel, c)
print(counts)

Expected behavior

Output a warning when we copy data to the host (if possible, with the source information)

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

• CUDA Quantum version:
• Python version:
• C++ compiler:
• Operating system:

Suggestions

No response

[schweitzpgi]

It seems like we maybe don't want a variable of type State to be used in general python expressions.

So after

c = np.array([1, 2j, 3, 4j, 5, 6j, 7, 8j], dtype=np.complex64)
state = cudaq.State.from_data(c)

the variable c is just a np.array to with one can do np.array sorts of things, but the variable state is an opaque box with no one really knows inside it.