The way it is written in UA, NO.

Note the use of is, instead of something like gains a +2 bonus

Any weapon you create using your Pact of the Blade feature is a +2 weapon. This invocation doesn’t affect a magic weapon you transformed into your pact weapon.

This is similar to the writing in Armors, in which your AC is something, and stacking armors (somehow) wouldn't stack bonuses.

The way it is written in XGtE, probably NO.

For the XGtE,

In addition, the weapon gains a +1 bonus to its attack and damage rolls

which would stack with another effect that said something on the lines of

In addition, the weapon gains a +2 bonus to its attack and damage rolls

except that it is explicitly states

unless it is a magic weapon that already has a bonus to those rolls.

and Pact Blade counts as a magical weapon (at least for resistances against non-magic weapons), as stated

You can use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it. You are proficient with it while you wield it. This weapon counts as magical for the purpose of overcoming Resistance and immunity to nonmagical attacks and damage.

While not explicitly stated that it counts as a magical weapon for the purpose of the bonus, it seems it is intended to not stack.

_{Note that the UA usually is not balanced (the point of it is to playtest and balance) and you should take care with using this content. From my experience, the writting was also not refined by then, as can be seen in the difference between the actual release and the UA version. You could suppose the intended purpose of what was written on UA is what is written on the official release, but that would not be RAW.}

The base damage dice for the weapon

For Mundane weapons, this is the standard die shown on the Weapons table on PHB p. 146 or here in the basic rules (thanks FenrirG!):

The Weapons table shows the most common weapons used in the worlds of d&d, their price and weight, the damage they deal when they hit, and any special properties they possess.

(emphasis mine)

If it is a magical weapon that has additional damage (like 2d6 fire from Flame Tongue) then that damage is included.

This is supported by Jeremy Crawford here in a discussion on Savage Attacks:

Savage Attacks: add 1 of a weapon's dice to a crit. again. Frost brand is a wpn. w/ 1d6 cold as 1 of its dice

So what doesn't count?

Pretty much everything else that isn't the above. If the die isn't included in the base stats of the weapon, then it's not included. This includes race/class/spell features that are above and beyond the base weapon stats that don't explicitly state a damage die is replaced (like with a monk (PHB, 78))

You can roll a d4 in place of the normal damage of your unarmed strike or monk weapon. This die changes as you gain monk levels, as shown in the Martial Arts column of the Monk table.

(emphasis mine)