Making filtering simpler

[diptanu]

1. Made executors report a list of function images
2. Making executors report Nvidia GPU card, memory available on a machine.
3. Using placement constraints to filter out images don't match labels such as python runtime, image name and version.
4. Made a copy on write in-memory cache of the executor state.

Next step

1. Add support for committing resources to functions

Test Plan -

1. Make 3 Compute Graphs which demands the same images
2. Make one function of each compute graph to request for exclusive access for hardware.
3. The scheduler needs to be able to select the right image and stick each function to one image. More than one function shouldn't occupy an image.