Adding option to use librosa backend.

Changes in the EstimatorBuilder to set attributes instead of returning tensors for the _build methods.
InputProvider classes to handle the different backend cases.
New method in Separator.

spleeter/utils/estimator.py

@@ -13,7 +13,7 @@ import tensorflow as tf
 from tensorflow.contrib import predictor
 # pylint: enable=import-error
 
-from ..model import model_fn
+from ..model import model_fn, InputProviderFactory
 from ..model.provider import get_default_model_provider
 
 # Default exporting directory for predictor.
@@ -59,14 +59,6 @@ def create_estimator(params, MWF):
     return estimator
 
 
-def get_input_dict_placeholders(params):
-    shape = (None, params['n_channels'])
-    features = {
-        'waveform': tf.compat.v1.placeholder(tf.float32, shape=shape, name="waveform"),
-        'audio_id': tf.compat.v1.placeholder(tf.string, name="audio_id")}
-    return features
-
-
 def to_predictor(estimator, directory=DEFAULT_EXPORT_DIRECTORY):
     """ Exports given estimator as predictor into the given directory
     and returns associated tf.predictor instance.
@@ -74,8 +66,10 @@ def to_predictor(estimator, directory=DEFAULT_EXPORT_DIRECTORY):
     :param estimator: Estimator to export.
     :param directory: (Optional) path to write exported model into.
     """
+
+    input_provider = InputProviderFactory.get(estimator.params)
     def receiver():
-        features = get_input_dict_placeholders(estimator.params)
+        features = input_provider.get_input_dict_placeholders()
         return tf.estimator.export.ServingInputReceiver(features, features)
 
     estimator.export_saved_model(directory, receiver)