Add abs calculations under precise_mode

This adds a second run of reference model under precise_mode
when test_desc.json contains a "compliance" dictionary which
contains a "mode" entry with value "dot product".

In this second run, abs_mode will be set to true, which causes:

1. evaluation will take absolute values of inputs for these operators:
	conv2d, conv3d, depthwise_conv2d, fully_connected,
	matmul, transpose_conv2d, fft2d, rfft2d
	reduce_sum, avg_pool2d
2. output files will have prefix "bounds_" prepended to them

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: I7070ecc7ead2d2ea3375c44663d653c6772b88e0