Attack Recipe Implementation - Leap

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Summary

Implements the attack method in "LEAP: Efficient and Automated Test Method for NLP Software", Xiao et al., 2023.

Adds:

• New attack recipe ("leap", leap_2023)
• New search method, a variant of ParticleSwarmOptimization (ParticleSwarmOptimizationLEAP). LEAP generates the initial population using Levy flight and Brownian motion. Then LEAP uses a new adaptive inertia weight update strategy for LEAP to optimize the search path in an exponentially growing text space.