[bazel] Add --config=shell for easier debugging (pytorch#79350)

Bazel quality of life improvement.

This change adds a new option `--config=shell` which allows you to drop-in the shell right before the `bazel run` command is executed. For example you will have ability to examine bazel sandbox this way, run things under gdb instead of a normal run and so on.

Example usage:

```
bazel run --config=shell //:init_test
```
Pull Request resolved: pytorch#79350
Approved by: https://github.com/dagitses

Author: vors

.bazelrc

@@ -31,6 +31,10 @@ build --copt=-isystem --copt=bazel-out/k8-fastbuild-cpu-only/bin
 # rules_cuda configuration
 build:cpu-only --@rules_cuda//cuda:enable_cuda=False
 
+# Definition of --config=shell
+# interactive shell immediately before execution
+build:shell --run_under="//tools/bazel_tools:shellwrap"
+
 # Disable all warnings for external repositories. We don't care about
 # their warnings.
 build --per_file_copt=^external/@-w

tools/bazel_tools/BUILD.bazel

@@ -0,0 +1,5 @@
+sh_binary(
+    name = "shellwrap",
+    srcs = ["shellwrap.sh"],
+    visibility = ["//visibility:public"],
+)

tools/bazel_tools/shellwrap.sh

@@ -0,0 +1,58 @@
+#!/bin/bash
+
+# This script is helpful in entering an interactive shell from a bazel build
+# before running a given bazel executable.
+# This can provide a quick way to explore the sandbox directory and filesystem.
+# Typical use is with
+#
+#     bazel run --run_under=//tools/bazel:shell_wrapper //:target
+#     OR
+#     bazel run --config=shell //:target
+
+shell='/bin/bash'
+rcfile='/tmp/pytorch_bazel_tools_shellwrap'
+while [[ $# -gt 0 ]] ; do
+    case "$1" in
+        --shell_bin_path)
+            # path for the shell executable
+            shell="$2"
+            shift 2
+            ;;
+        --rcfile)
+            # path for the file used to write the environment
+            rcfile="$2"
+            shift 2
+            ;;
+        *)
+            # remaining arguments are part of the command for execution
+            break
+            ;;
+    esac
+done
+
+if ! tty -s; then
+    echo 'A tty is not available.'
+    echo "Use \`bazel run\`, not \`bazel test\`."
+    exit 1
+fi
+
+NOCOLOR='\033[0m'
+YELLOW='\033[1;33m'
+
+# store the environment in a file
+export PYTORCH_SHELL_COMMAND=$*
+echo "alias run=\"$*\"" > "$rcfile"
+echo "PS1='\s-\v\$ '" >> "$rcfile"
+
+echo =====
+# print the execution command (command is yellow)
+echo -e "alias run=${YELLOW}$PYTORCH_SHELL_COMMAND${NOCOLOR}"
+echo =====
+
+echo "Entering interactive shell at the execution root:"
+
+# quote escape all the arguments to use as a single input string
+cmd="'$shell' --noprofile --rcfile '$rcfile'"
+
+# run the command in a script psuedo terminal and dump to null
+/usr/bin/script -c "$cmd" -q /dev/null