Normalizing flow example (#133)

* Implement normalizing flow Real NVP example

* Add requirements and basic usage to normalizing flow example

* Minor changes to README in normalizing flow example

* Remove trailing commas in function arguments for unified formatting in flows example

* Fix minor typos, add some annotations

* format + nits in README

* readme fix

* mov, minor changes in main, copywright

* remove debug

* fix

* Simplified class structure in distributions; better code re-use in bijectors

* Remove rogue space

* change name again

* nits

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Co-authored-by: Awni Hannun <awni@apple.com>

normalizing_flow/README.md

@@ -0,0 +1,52 @@
+# Normalizing Flow
+
+An example of a normalizing flow for density estimation and sampling
+implemented in MLX. This example implements the real NVP (non-volume
+preserving) model.[^1] 
+
+## Basic usage
+
+```python
+import mlx.core as mx
+from flows import RealNVP
+
+model = RealNVP(n_transforms=8, d_params=4, d_hidden=256, n_layers=4)
+
+x = mx.random.normal(shape=(32, 4))
+
+# Evaluate log-density
+log_prob = model.log_prob(x=x)
+
+# Draw samples
+x_samples = model.sample(sample_shape=(32, 4))
+```
+
+## Running the example
+
+Install the dependencies:
+
+```
+pip install -r requirements.txt
+```
+
+The example can be run with:
+```
+python main.py [--cpu]
+```
+
+This trains the normalizing flow on the two moons dataset and plots the result
+in `samples.png`. The optional `--cpu` flag can be used to run the example on
+the CPU, otherwise it will use the GPU by default.
+
+For all available options, run:
+
+```
+python main.py --help
+```
+
+## Results
+
+![Samples](./samples.png)
+
+[^1]: This example is from [Density estimation using Real NVP](
+  https://arxiv.org/abs/1605.08803), Dinh et al. (2016)