For example "lasers are being used to treat different skin conditions". It is a passive form without a subject. Please guide.
Short answer: Yes, it's fine. Totally fine.
If you're interested:
It's the progressive passive; a combination of the progressive aspect and the passive voice. It does have a subject, which is lasers. But being passive the subject is the patient rather than the agent of the verb.
There're two passive forms in English.
This latter uses the progressive aspect in a passive construction.
The progressive passive (and later the related progressive-of-be, which is another form of "are being") was one of the bêtes noires of some 19th Century grammarians, but is now completely accepted. (Bar some general dislike of the passive in some quarters, though even they tend to dislike it rather than claim it is ungrammatical).
(The other passive construction is much older, and raised no such objections).
There's a degree of irony, in that while some complaints from that time that persist to this day are about forms that were in the language for many centuries before some people decided they were incorrect, this form actually was quite new, and nobody complains about it.
A big part of the reason is that we don't have as much use of the passival any more. Once upon a time we could say something like "lasers are using to treat different skin conditions" to mean the same thing. Only we couldn't really because Einstein didn't develop the theory behind lasers until 1917, and that form was pretty dead then. It would be considered ungrammatical today, though the remnant persists in some middle voice constructions ("the dinner was cooking", "the document was printing").
Yes, it's grammatical. It's a passive construction in which the subject is lasers. You perhaps meant that there is no agent (we don't know who the lasers are being used by), but that is not unusual in a passive construction.
It is grammatically correct but it has not quite the same meaning as
which may just explain the function of lasers.
indicates they are actually in operation.
Yes, it's correct. As Barri mentioned it's a passive construction. We use the passive construction when when are more interested in the action. For example:
In the second example, you are merely interested in saying that they were killed. You either don't know who killed them or are merely interested in the action itself.