offloading_algo.py: fix device_only network cost + move itertools import

[ff225]

Priority: Medium

Correctness and cleanup in the offloading algorithm (same file).

Tasks

• device_only_evaluation adds a network cost (layer_data_size / avg_speed) even though a fully-local inference transmits no intermediate data; this can distort the comparison between strategies. While here, review the asymmetry with edge_only, which uses a 0 network cost and does not model sending the input to the edge.
• Move import itertools out of the hot-path method mixed_computation_evaluation to module top-level.

Files

• src/server/offloading_algo/offloading_algo.py

From the Feature 042 codebase review (recommendations #8, #7).