My ipad pro is locked on activation lock, i used it barely for 3 weeks ,seller forgot the icloud and password

If the iPad disabled or Activation Locked, unless you can independently obtain assistance from the previous owner, the iPad is (and will remain) useless to you.

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

Many so-called “second hand” devices are in fact stolen goods. The Activation Lock is intended to ensure that devices can only be reset by the original or legitimate subsequent owner. If properly reset before being sold to a new owner, there won’t be an Activation Lock - and the device can be used again without issue. As stolen devices will be Activation Locked, they have no value to the thief; they will hopefully learn and not bother attempting to waste time and effort in attempting to steal and resell a device that has no value.

The All Too Common SAD Reality of Buying … - Apple Community

If you have purchased a pre-owned device that has not been properly prepared for reuse - and legitimately resold - this is not the fault of Apple. Buyers should be mindful - and have responsibility - to ensure that they are acquiring a pre-owned device from a seller who has legitimate title to the item being sold.

Here is what Apple suggests when purchasing a previously used device:

Check for Activation Lock before you buy a device from someone else

Before you buy an iPhone, iPad, or iPod touch, from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account. Follow these steps to check if the device is ready to use: 

1. Turn on and unlock the device. 
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, contact them as soon as possible and ask them to use a browser to remove the device from their account. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.

You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.

If the iPad was not Activation Locked when purchased, you would not have this problem now.

Your initial post, as titled, indicates that the iPad is locked with AppleID credentials that are known to have been “forgotten” - this tale being a typical ploy to encourage and fool the unwary into purchasing a device that will ultimately, as now, be useless to the subsequent buyer.

Unfortunately, you have been the victim of either intentional fraud - or just bad luck. Unfortunately, there is absolutely nothing that you can do other than to seek redress from whoever sold you the device - who is now likely long gone and uncontactable.

Sorry, but Apple - and end-user contributors of this Support Community - cannot help you resolve the unresolvable.

You decided to buy a second-hand device from the grey (very grey) market. I’m afraid that’s on you, not Apple.