Add context to VAE-IAF

[clementchadebec]

New features

• Add the possibility to add a context vector h to MADE. This is made to fit to Fig.2 in https://arxiv.org/abs/1606.04934 where the IAF are parametrized by a context vector h.
• Add a context_dim argument in the MADEConfig, IAFConfig, MAFConfig, IAFSamplerConfig, MAFSamplerConfig and VAE_IAF_Config.
• :warning: if the argument context_dim is different from None the VAE_IAF model expects the encoder to output a context vector h in addition to the parameters of the posterior distribution.

Minor changes

• rename n_made_blocks --> n_blocks in IAF and MAF configs
• add automatic context layer in benchmarks neural nets.