Retrieve intermediate adapt vqe information.

[justinlietz]

Along with returning the final results, it would be useful to somehow access the result at intermediate steps of the calculation. Perhaps an optional list parameter could be supplied where the results from each step could be appended. Alternatively, a file could be provided where the results are stashed.

[ikkoham]

I thought it might be a good idea to allow saving and loading as a feature on the cudaq primitive side.
For example,

import os
os.environ["SAVE_PRIMITIVES"] = True
# optional? os.environ["SAVE_PRIMITIVES_DIR"] = "./some_nice_directory/"

result = cudaq.observe(kernel, operator)
print(result.primitive_id) # prints some id

# retrive from id
result2 = cudaq.retrieve_primitives(result.primitive_id)

assert result == result2

# you can retrieve the job from the filename
result3 = cudaq.retrieve_primitives("./filename.pr")

assert result == result3

and user can get the inputs like

# result has the input kernel and operator
assert kernel == result3.kernel
assert operator == result3.spin_operator

# or the input could be handled by a separate class.
inputs = cudaq.get_primitive_inputs(result.primitive_id)
assert kernel == inputs.kernel
assert operator ==  inputs.spin_operator