python: add initial cudaFromGstSample support

[aconchillo]

This patch adds initial bindings support for GStreamer. It currently only
provides a function cudaFromGstSample.

cudaFromGstSample expects a GstSample as a single argument. The GstSample
needs to be in NV12 format and stored in host memory (this means there's no
zero-copy). The function will then copy the frame to CUDA memory and convert
it to RGBA (float4) directly in the GPU.

A usage example could be:

def on_new_sample(appsink):
    sample = appsink.emit('pull-sample')

    img, width, height = jetson.utils.cudaFromGstSample(sample)

    # classify the image
    class_idx, confidence = net.Classify(img, width, height)

When using GStreamer for, let's say, decoding a video encoded in H.264, it is
very likely that OpenMAX GStreamer plugins are used (i.e. omxh264dec). This
will create a video/x-raw(memory:NVMM) frame that is mapped into NVMM
memory (i.e. DMA buffer). Unfortunately, it's not straight forward to convert
NVMM memory to CUDA, that's why cudaFromGstSample expects the frame to be
stored in the host memory. To copy an NVMM mapped frame to host memory, your
pipeline needs to include the nvvidconv plugin which will convert an NVMM
mapped frame to a regular video/x-raw frame.

[blitzvb]

nice!

do you have an example including the GstSample ? which package do you use for that on a nano?

thanks.

[dusty-nv]

Thanks for the contribution @aconchillo!

If you have the code of reading video using this (presumably using appsink), would like to merge it all.

[aconchillo]

New examples PR dusty-nv/jetson-inference#389

[aconchillo]

Added a new VideoLoader class. Not sure if I also need to added to the Jetson.Utils package, I don't know the difference.

[aconchillo]

There was an issue with nvv4l2decoder negotiating caps with videorate. This is now fixed and works with JetPack 4.2.1. nvv4l2decoder now has a higher ranking (therefore chosen) than omxh264dec.

[aconchillo]

Renamed VideoLoader to VideoSource since I'm planning to add a VideoSink at some point.

[aconchillo]

Added a VideoSink class useful to create video files. Also added GstBuffer to numpy array conversions.