The fleshy party of the fruit of theobroma cacao is is supposed to be sweet and pleasant. However, it does not taste like chocolate.
Chocolate is made from the nibs or seeds within the fruit of theobroma cacao, after it is fermented ground, and processed, and is in no way sweet.
The nibs themselves are very low in sugar, and contain alkaloids (such as caffeine or theobromine) which are bitter. This probably evolved to discourage predation or disease to the seeds themselves.
This is very similar to the way peach kernels (the seed within the pits) are not terribly sweet, although they certainly grow within a fruit that is.
So no—there is no such thing as naturally sweet chocolate.
A related tree, Cupuaçu (theobroma grandiflorum) is cultivated more specifically for its fruits, which are sweet and said to taste something like pineapple. Again, the fleshy fruit is not analogous to the seed which eventually becomes chocolate.