Multiple interesting comments (including content in answers) on this one. e.g., I liked this part from Dave Kaye's comment:
There is no guide. Do what feels right and communicate it clearly so your students understand.
The "question" section asked these questions:
Do any such resources exist? Can anyone point me in the right direction? If these don't exist, why not?
Those are excellent questions to which I am not directly answering. However, I do address the topic of the title question, "How do teachers learn to grade?"
Growing up, I heard that before a person becomes a teacher, they go through a process of being a "student teacher" where they learn things like this.
Well, let me tell you my experience. (I imagine this will raise some eyebrows.)
A college reached out to me and asked me to become an instructor. I agreed. I don't recall ever being taught how to assign grades. So I did things by trial and error.
It turns out that the college department I joined was failing in multiple ways. The drop-out rate was high. For those that didn't drop out, the department chair taught courses and gave students solid As, but they obtained degrees without sufficient skill/knowledge. I decided to do something about this, and I gave lower grades when I determined they were deserved.
Sometimes, grades would end up being too low. A student once said that I took the Gandalf approach to making tests. ("You shall not pass!") I recall the time in my second set of classes where the top grades on the midterm were something like an 82, a 76, and a 58. Clearly I didn't want to flunk the vast majority of the class (which might have triggered numerous more drop-outs). My tests were not too hard: The intent of this department was to train people well enough to take industry certifications, and those were slightly harder than what I gave. So I had to be stern and have them rise to the challenge.
At the end of every quarter, after final exams were collected and graded, I looked at the grade of every student. Then I would make adjustments to the value of assignments and tests, as needed, so that the final grades made sense to me. I had some students who were clearly trying the hardest, succeeding at tests the best, and learning the most. I figured they ought to get an A, and any deviation from that would be a failure of the assessment system, not a failure of the student. So I made sure the people who deserved top marks got them, and I looked at the grades of every other student and thought about what their grade looked like and how well they were learning the material. I made sure that nobody got a lower grade than I thought they deserved, and the vast majority of the students got exactly what grade I thought they deserved. If that wasn't true, then I adjusted some assignment/test weights equally across the board.
In the end, I was usually satisfied with every student's grade, but sometimes a couple of students would end up with a grade that was a notch (or maybe two) higher than I intended, and I couldn't adjust their grade without making someone else look wrong (giving someone else a grade lower than deserved). In those cases, I just figured that the student, whom I thought was performing a bit low, must've actually done a bit better than I thought, and so I finalized things with them getting a slightly higher grade than I would have thought. (I figured that even if a student got answers right due to luck, I had no grounds to penalize them.)
There was some grumbling about people not getting solid A's as easily, and the increased workload I may have been expecting, and tests being much harder. Nobody ever actually challenged a grade of mine, or asked me why they got the grade that they did.
I view the results of my efforts as entirely positive. Grades were doing a better job of reflecting reality. I feel like I improved things as much as possible without causing so much student frustration as to skyrocket the dropout rate. Actually, as my involvement increased (particularly as I led the department, since the department chair left within 90 days of my start), drop-outs happened less. More accuracy, stronger resulting students, and better critical numbers all seemed like good things.
I do recall the college president speaking to the instructors, showing that too many As were being given out. In the entire college, over half of all grades were As, and most of the rest were Bs. He told us to make such good grades harder to get. (I happily thought about how I was doing exactly that.) Besides that one point of instruction, I don't recall other teaching about how to assign the final grades. There were some guidelines that had to be followed (relative weights documented on the syllabus, rubrics for some specific college-wide assignments), but we still had some flexibility (which I definitely utilized) and ultimately it was up to the instructor's authority to determine what made sense.
So, in my case, the answer was largely trial-and-error (but fixing the errors before the finalized grades got posted). I'm sure that at another university I've been a part of, graders had specific rubrics about everything and they had to follow those very closely, and be able to justify how every grade fit within the rubric they were assigned to, with much less flexibility.
Like another answer of mine, the ultimate conclusion is that specifics vary A LOT between different organizations (including different institutions, and possibly between different departments. Different instructors are very likely to have significantly different experiences. Although I'm sure people have written books on the subject (as well as all kinds of other subjects on the planet), that doesn't mean that instructors actually go through any sort of centralized/standardized training in practice. I know that the state I live in has some standardized test for teachers of Kindergarten through 12th grade, but ironically, there can be a lot more variance in the higher education.