Was so-called “early PIE” a single language without dialects or a wide continuum of dialects? If it was a dialect continuum, then probably when did the “common” PIE split up into dialects?
"Early PIE" refers to the stage before the split of Anatolian and the rest ("late PIE"). I don't think that we really know much about that stage of the proto-language because Anatolian is very divergent from the rest. Having only a binary split there is not much wiggle room for proto-language dialectology. In real life we expect any language to develop dialects, but in this case we cannot know them.
It depends a little on what you mean by Early PIE, whether you mean the product of our reconstructions, or the actual historical language we are aiming to study
It is extremely unusual for a language to exist without dialectal variation, and the few examples there are are almost all small and isolated populations
Early PIE was certainly not isolated, and doesn't seem to have had an especially small speaker population. As such, it is almost certain that the language ancestral to all Indo-European languages (including Anatolian) as actually spoken was either a dialect continuum or already had distinct dialects
Jk's answer already well addresses the question of Early PIE as we can reconstruct it
So many possible answers: a) we can never know. b) was Early-PIE an actual language or is it just a reconstruction reflecting elements of a language spoken at various times? Remember you could never reconstruct classical latin with all its case system from modern romance languages. c) the normal development of languages is that a single dialect spreads across an area becoming a dialect continuum which then breaks into different languages when dialects are split off from each other (by migration for example) or when specific dialects gain a superior status (by becoming official dialects of states for example). d) at the time that Indo-European was spoken on the steppes it may have been spoken over a considerable area so it would make sense its being a dialect continuum. e) if you agree with Kristiansen that Indo-European was adopted by the Yamna steppe culture from the Maykop culture than it may be that a single language was borrowed from the Maykop culture which became a dialect continuum on the steppes.