Syn failed for 3Layer with sublayer

Created by: zhenbinwu

The current master branch of the hls4ml can't synthesize the 3layer Model.

ERROR: [XFORM 203-103] Array 'mult.V' (/data/benwu/HLS4ML_2018/hls4ml/nnet_utils/nnet_layer.h:56): partitioned elements number (2048) has exeeded the threshold (1024), which may cause long run-time.
ERROR: [HLS 200-70] Pre-synthesis failed.
command 'ap_source' returned error code
    while executing
"source [lindex $::argv 1] "
    ("uplevel" body line 1)
    invoked from within
"uplevel \#0 { source [lindex $::argv 1] } "

INFO: [Common 17-206] Exiting vivado_hls at Fri Jul 13 12:00:25 2018...

The error can be trace to https://github.com/hls-fpga-machine-learning/hls4ml/pull/62/files#diff-b37c065f136460b015788b96b5c25102L52