Single config attribute for weight tying (#42815)
* merge two attr into one
* delete tie encoder decoder
* one more
* mt5
* skip tests when tying is hardcoded
* change test value to True, so we don't have to adjust hardcoded configs
* awful decision in t5 to support two variants
* delete my comment
* not copied anymore
* skip
* they all had a shared embedding which was hardcoded, force it
* force it in umt5 also, my model won't work otherwise :(
* skip the key
* dont't force if official weights set it to True
* skip one test and fix teh other
