demonstrate tensorgraph layer constructions

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### TensorGraph Layers and TensorFlow eager

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 In this tutorial we will look at the working of TensorGraph layer with TensorFlow eager.

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``` python

import tensorflow as tf

import tensorflow.contrib.eager as tfe

```

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    /home/nitin/anaconda3/envs/deepchem/lib/python3.5/site-packages/h5py/__init__.py:36: FutureWarning: Conversion of the second argument of issubdtype from `float` to `np.floating` is deprecated. In future, it will be treated as `np.float64 == np.dtype(float).type`.

      from ._conv import register_converters as _register_converters

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After importing neccessary modules, at the program startup we invoke `enable_eager_execution()`.

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``` python

tfe.enable_eager_execution()

```

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### Importing TensorGraph Layers

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``` python

import deepchem as dc

from deepchem.models.tensorgraph.layers import Dense, Conv1D

```

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``` python

# Initialize a tensor

inputs = tf.constant([[1.0, 2.0], [4.0, 5.0]])

```

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``` python

# This will create a Dense layer

dense_layer = Dense(3)

```

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The `create_tensor()` function doesn't perform any tensor operation but is a method of `Dense`. It takes in a tensor object as a parameter.

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``` python

x = dense_layer.create_tensor(in_layers = [inputs])

print(x)

```

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    tf.Tensor(

    [[-0.22144215  0.17204109 -0.91569924]

     [-1.1559541   0.81352365 -0.9357872 ]], shape=(2, 3), dtype=float32)

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The above function call performs the same action as the below. This is because `create_tensor()` is invoked by `__call__()` object. This gives us an advantage of directly passing the tensor as a parameter while constructing a TensorGraph layer.

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``` python

y = Dense(2)(inputs) # creates a layer that outputs a tensor of shape=(2x2)

print(y)

```

%% Output

    tf.Tensor(

    [[  1.2503337  -4.140855 ]

     [  2.821608  -10.878647 ]], shape=(2, 2), dtype=float32)

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### Example

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The following code snippet shows a basic architechture of how TensorGraph layer can be created in TensorFlow eager mode.

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Import all the necessary modules and `enable_eager_execution()`.

```python

import tensorflow as tf

import tensorflow.contrib.eager as tfe

import deepchem as dc

from deepchem.models.tensorgraph.layers import Dense

tfe.enable_eager_execution()

x = [[4.]]

output = Dense(3)(x)

print(output)

```

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`Dense` layers are one of the layers defined in Deepchem. Along with it there are several others like `Conv1D`, `Conv2D`, `conv3D` etc. We show constructing a `Conv1D` layer below.

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Basically this layer creates a convolution kernel that is convolved with the layer input over a single spatial (or temporal) dimension to produce a tensor of outputs.

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``` python

conv_layer = Conv1D(2, 1)

z = conv_layer.create_tensor(in_layers = [inputs])

print(z)

```

%% Output

    tf.Tensor(

    [[[0.53595114 0.15551412]

      [1.0719023  0.31102824]]

     [[2.1438046  0.6220565 ]

      [2.6797557  0.7775706 ]]], shape=(2, 2, 2), dtype=float32)

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### Gradients

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``` python

```