Enable specifying LR Schedule in model config

[amanchoudhri]

Closes #26, closes #27.

Adds subdict LR_PARAMS to the OPTIMIZATION section of config files, expects param SCHEDULER. Note a breaking change: changes expected param name MAX_LEARNING_RATE to INITIAL_LEARNING_RATE.

Example sequential optimization subconfig:

"OPTIMIZATION": {
    "NUM_EPOCHS": 500,
    "OPTIMIZER": "SGD",
    "MOMENTUM": 0.9,
    "WEIGHT_DECAY": 1e-05,
    "CLIP_GRADIENTS": true,
    "INITIAL_LEARNING_RATE": 0.02,
    "SCHEDULERS": [
           {
              "SCHEDULER_TYPE": "COSINE_ANNEALING",
              "MIN_LEARNING_RATE": 0.0,
              "NUM_EPOCHS_ACTIVE": 100,
           },
           {
              "SCHEDULER_TYPE": "EXPONENTIAL_DECAY",
              "MULTIPLICATIVE_DECAY_FACTOR": 0.5,
           },
     ]
}

Example subconfig if only one scheduler is desired:

"OPTIMIZATION": {
    "NUM_EPOCHS": 500,
    "OPTIMIZER": "SGD",
    "MOMENTUM": 0.9,
    "WEIGHT_DECAY": 1e-05,
    "CLIP_GRADIENTS": true,
    "INITIAL_LEARNING_RATE": 0.02,
    "SCHEDULERS": [
           {
              "SCHEDULER_TYPE": "REDUCE_ON_PLATEAU",
              "MULTIPLICATIVE_DECAY_FACTOR": 0.5,
              "PLATEAU_DECAY_PATIENCE": 25,
           },
     ]
}