experiments: store experiments in refs/exps instead of heads (#4910)

* experiments: commit to refs/exps/... instead of branch heads

* update exp show

* restore checkpoint_resume kwarg

* refactor new() to use experiments stash

* experiments: use refspecs to populate and collect executors

* experiments: use new stash and merge on exp checkout

* experiments: fix executor chdir

* experiments: update run and show

* skip checkout after reproduce

* checkpoints: use experiments refspecs

* check if workspace was stashed before applying

* exp show: unify checkpoint/standalone experiment collection

* git: move iter_refs to backend

* update for scm changes

* experiments: replace exp checkout in favor of exp apply

* git: handle rev_parse errors in resolve_commit

* update refs containing and baseline check

* fix sigint handling for checkpoints

* update checkpoint exists behavior

* fix checkpoint baseline behavior

* update tests

* experiments: update gc

* update show tests

* fix type checking errors

* update diff

* scm: add diff, describe

* experiments: generate human readable names

* exp run/resume: allow user specified exp name with -n/--name

* update tests

* experiments: add `exp branch` for promoting experiment to full branch

* fix test

* fix missing file

* experiments: explicitly close scm instances before cleaning up tmp dir

dvc/command/experiments.py

@@ -156,7 +156,8 @@ def _collect_rows(
                 else:
                     tree = "└──"
                 new_checkpoint = True
-            row.append(f"{tree} {queued}{rev[:7]}{parent}")
+            name = exp.get("name", rev[:7])
+            row.append(f"{tree} {queued}{name}{parent}")
 
         if not no_timestamp:
             row.append(_format_time(exp.get("timestamp")))
@@ -352,12 +353,12 @@ def run(self):
         return 0
 
 
-class CmdExperimentsCheckout(CmdBase):
+class CmdExperimentsApply(CmdBase):
     def run(self):
         if not self.repo.experiments:
             return 0
 
-        self.repo.experiments.checkout(self.args.experiment)
+        self.repo.experiments.apply(self.args.experiment)
 
         return 0
 
@@ -461,6 +462,7 @@ def run(self):
             try:
                 self.repo.experiments.run(
                     target,
+                    name=self.args.name,
                     queue=self.args.queue,
                     run_all=self.args.run_all,
                     jobs=self.args.jobs,
@@ -532,6 +534,16 @@ def run(self):
         return 0
 
 
+class CmdExperimentsBranch(CmdBase):
+    def run(self):
+        if not self.repo.experiments:
+            return 0
+
+        self.repo.experiments.branch(self.args.experiment, self.args.branch)
+
+        return 0
+
+
 def add_parser(subparsers, parent_parser):
     EXPERIMENTS_HELP = "Commands to display and compare experiments."
 
@@ -653,20 +665,22 @@ def add_parser(subparsers, parent_parser):
     )
     experiments_show_parser.set_defaults(func=CmdExperimentsShow)
 
-    EXPERIMENTS_CHECKOUT_HELP = "Checkout experiments."
-    experiments_checkout_parser = experiments_subparsers.add_parser(
-        "checkout",
+    EXPERIMENTS_APPLY_HELP = (
+        "Apply the changes from an experiment to your workspace."
+    )
+    experiments_apply_parser = experiments_subparsers.add_parser(
+        "apply",
         parents=[parent_parser],
         description=append_doc_link(
-            EXPERIMENTS_CHECKOUT_HELP, "experiments/checkout"
+            EXPERIMENTS_APPLY_HELP, "experiments/apply"
         ),
-        help=EXPERIMENTS_CHECKOUT_HELP,
+        help=EXPERIMENTS_APPLY_HELP,
         formatter_class=argparse.RawDescriptionHelpFormatter,
     )
-    experiments_checkout_parser.add_argument(
-        "experiment", help="Checkout this experiment.",
+    experiments_apply_parser.add_argument(
+        "experiment", help="Experiment to be applied.",
     )
-    experiments_checkout_parser.set_defaults(func=CmdExperimentsCheckout)
+    experiments_apply_parser.set_defaults(func=CmdExperimentsApply)
 
     EXPERIMENTS_DIFF_HELP = (
         "Show changes between experiments in the DVC repository."
@@ -829,11 +843,38 @@ def add_parser(subparsers, parent_parser):
     )
     experiments_gc_parser.set_defaults(func=CmdExperimentsGC)
 
+    EXPERIMENTS_BRANCH_HELP = "Promote an experiment to a Git branch."
+    experiments_branch_parser = experiments_subparsers.add_parser(
+        "branch",
+        parents=[parent_parser],
+        description=append_doc_link(
+            EXPERIMENTS_BRANCH_HELP, "experiments/branch"
+        ),
+        help=EXPERIMENTS_BRANCH_HELP,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
+    experiments_branch_parser.add_argument(
+        "experiment", help="Experiment to be promoted.",
+    )
+    experiments_branch_parser.add_argument(
+        "branch", help="Git branch name to use.",
+    )
+    experiments_branch_parser.set_defaults(func=CmdExperimentsBranch)
+
 
 def _add_run_common(parser):
     """Add common args for 'exp run' and 'exp resume'."""
     # inherit arguments from `dvc repro`
     add_repro_arguments(parser)
+    parser.add_argument(
+        "-n",
+        "--name",
+        default=None,
+        help=(
+            "Human-readable experiment name. If not specified, a name will "
+            "be auto-generated."
+        ),
+    )
     parser.add_argument(
         "--params",
         action="append",