This commit makes following changes:
* deleted most atrous_conv2d code to reuse existing tf.nn.convolution function
* added atrous_conv1d and atrous_conv3d with similar API as atrous_conv2d
* Added support for variable rate per dimension, e.g. for atrous_conv2d
  rate=2, or rate=[2,1] does different things. Former is equal to
  rate=[2,2]. rate_i determines dilation_rate in dimension i.
* added strides support with same API as in tf.nn.convolution function

This commit makes more code deletions then additions. However
documentation per function makes it appear large.

Test Plan:

Some simple tests to verify I haven't screwed something up:

```
A = np.array([1, 2, 3, 4, 5, 6], dtype=np.float32).reshape(1, 6, 1)
print(A)

kernel = np.array([100, 10, 1], dtype=np.float32).reshape(3, 1, 1)

with tf.Session() as sess:
    print(sess.run(tf.nn.atrous_conv1d(A, kernel, padding='SAME', rate=[2])))
```

```
B = np.arange(16, dtype=np.float32).reshape(1, 4, 4, 1)
kernel = np.array([1000, 100, 10, 1.0], dtype=np.float32).reshape(2, 2, 1, 1)

with tf.Session() as sess:
    a = sess.run(tf.nn.convolution(B, kernel, padding='SAME', dilation_rate=np.array([2, 2])))
    b = sess.run(tf.nn.atrous_conv2d(B, kernel, rate=2, padding='SAME'))
    print(np.allclose(a, b))
    print(a)
    print(b)
```

```
C = np.arange(4**3, dtype=np.float32).reshape(1, 4, 4, 4, 1)
kernel = (10**np.arange(8, 0, -1, dtype=np.float32)).reshape(2, 2, 2, 1, 1)

with tf.Session() as sess:
    a = sess.run(tf.nn.conv3d(C, kernel, strides=[1, 1,1,1,1], padding='SAME'))
    b = sess.run(tf.nn.atrous_conv3d(C, kernel, rate=1, padding='SAME'))
    print(np.allclose(a, b))
    print(a)
    print(b)
```

Also running atrous_conv2d unit tests to verify backward compatibility.

Deleted atrous_conv1d and atrous_conv3d.
Added note in documentation for atrous_conv2d that convolution should be
used instead.