Stop pipeline fitting after the last block with fit method

[sarahmish]

Right now, an MLPipeline produces the output of all primitives (blocks) within the pipeline (with the exception of the last primitive and designated output).

To save time, we can stop producing the output of blocks once we have reached the last block that can be fitted. To make these changes we can make the following changes:

• figure out the last block that needs to be fitted in the pipeline and call it last_fit_block.
• In L#780, check if the current block is after the last_fit_block. If true, then there is no need to produce the output of the current block.

if (block is before self._last_fit_block) or (block_name in output_blocks):
    self._produce_block(
        block, block_name, context, output_variables, outputs, debug_info)