Added autoregressive heads and sampling

modules/generative_head/composite_head_A.py

modules/generative_head/convolution_head_A.py

modules/generative_head/double_substitution_head.py

modules/generative_head/head_factory.py

modules/generative_head/substitution_head.py

modules/utils/__init__.py

modules/utils/block_convolution.py

sample/token_generator/basic_generator.py

sample/token_generator/composite_generator.py

sample/token_generator/double_substitution_generator.py

sample/token_generator/substitution_generator.py

sample/token_generator/token_generator_factory.py