optimize size and time using "--no-cache-dir"

Using "--no-cache-dir" flag in pip install ,make sure dowloaded packages
by pip don't cached on system . This is a best practise which make sure
to fetch ftom repo instead of using local cached one . Further , in case
of Docker Containers , by restricing caching , we can reduce image size.
In term of stats , it depends upon the number of python packages
multiplied by their respective size . e.g for heavy packages with a lot
of dependencies it reduce a lot by don't caching pip packages.

Further , more detail information can be found at

https://medium.com/sciforce/strategies-of-docker-images-optimization-2ca9cc5719b6

Signed-off-by: Pratik Raj <rajpratik71@gmail.com>

examples/keras/Dockerfile

@@ -36,5 +36,5 @@ RUN cd /usr/local/bin && \
     ln -s /usr/bin/pip3 pip && \
     pip install --upgrade pip setuptools
 
-RUN pip install numpy==1.16.5 tensorflow-gpu==1.15.0 contextlib2==0.6.0 tqdm==4.36.1
-RUN pip install tensorflow-datasets==1.2.0
+RUN pip install --no-cache-dir numpy==1.16.5 tensorflow-gpu==1.15.0 contextlib2==0.6.0 tqdm==4.36.1
+RUN pip install --no-cache-dir tensorflow-datasets==1.2.0