I am not using vdd and gnd as pins in schematic(instead using instances from analoglib).Now after layout is done and config view is done,the post layout simulation is not considering the vdd and gnd(therefore shows different output waveform).If I use the vdd and gnd pins(in scematic) than i get the correct waveform, however this makes inverter symbol with four legs.
The pins on the layout could have global names, and then it should work. However, the best solution would be to use inherited connections. If you create pins on your schematic for vdd and vss using a "netExpr" (defaulting to (say) vdd! and vss!), controlled by a properties vdd and vss respectively, and also use Layout XL to generate the layout - you'll have similar pins on the layout, and also on any resulting extracted view.
There's then no need to have the pins on the symbol - you can keep that having just the input and output pins. You'll also then have the ability to reuse the inverter in various supply domains by adding netSet properties at a higher level to override the values of the supplies.
This is well covered in the documentation - probably search for "inherited connections" or "netExpr" or something like that.