create README for model_definitions module

Author: kschuerholt

modules/model_definitions/README.md

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+# Model Definition Module
+
+This module contains the code for all models used in this work. 
+> ```def_net``` contains the model definition for the small MLP or CNN models in our own model zoos.
+> ```def_simclr_ae_module``` contains the training class for our neural representation learning models.
+> ```def_simclr_ae_trainable``` contains the ray.tune trainable wrapper around the module
+> ```./components/``` contains encoder and decoder architectures, as well as the different encodings and the loss definitions.
+> ```./downstream_tasks/``` contains a wrapper class around the downstream tasks as well as the baseline models.