1.During the filament change, the extruder will extrude an extra length
of grap_length for the corresponding detection, so the purge volume can
reduce this length.
jira:NEW
Signed-off-by: xun.zhang <xun.zhang@bambulab.com>
Change-Id: I14f55a3200c1dd48b1603e50280d0c29e1319ebe